Tongue twisters are such a fun twist on coding challenges! It’s cool to see how creative solutions keep the code short and sharp. If you’re looking for some free online entertainment during a break, this might be worth checking out.

AWK, 163 bytes

END{print"S"(a="he sells ")(b="sea"(c="shells"))" by the "(d="seashore")",\nThe "c" s"a"are "b","(e=" I'm sure")".\nSo if s"a b" on the "d",\nThen"e" s"a d" "c"."}

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Vyxal 3.0.0beta.1, 77 bytes

"dyrΦ<ᶲrᵞjvG#IZᶳ²∵nᶨ₂ᶜ8G6OẠ′zḃṁɠ₉EFḟih←Xᵞᶻ1d²i)`⁰!⌐=x‴¶ᶢ↓•ƓΦ…ᶢɾʀ5ḋỌḄcGᵒıɾᶪ×‴”

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This is more complicated than just calling your language's print equivalent.

Is it now.

Simply a single compressed string. No string formatting, no substitutions. Just one string.

Splinter, 150 bytes

I{ECA}H{\ }G{\r\e}F{H\I\'\mC\uG}E{\h\e}D{H\tEB\oG\,\
\TE}C{H\s}B{C\e\a\s\h}A{\e\l\l\s}\SIBAH\b\yDC\hACIH\aGBA\,F\.\
\S\oH\i\fCIBAH\o\nD\nFCIB\oGC\hA\.

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Deadfish~, 1216 1196 bytes

iiissiici{ii}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddd{i}ccddd{i}c{{d{{i}}}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cdddcddd{i}ccddd{i}c{{d{{i}}}}c{d{i{i}}}ciii{ii}ci{i{d}}c{iiii{iii}}cdd{d}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cddd{i}ciiicddd{d}cd{{ii}}c{d{{i}}}cdsiiic{ii}cdddci{iii{d}}ciii{dd{i}}cd{d}cdddcddd{i}ccddd{i}c{{d{{i}}}}ciii{dd{i}}cd{d}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddd{i}ccddd{i}c{{d{{i}}}}ci{ii{iii}}cddd{ii}cddd{d}ci{iii{d}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cdddcddd{i}ccddd{i}cd{iii{d}}cdd{d}ci{iiii}c{d{{i}}}c{ddd{i}}cd{dd{ii}}ciii{dd{i}}ciicdddcddd{d}ci{{ii}}c{ii{ii}}cdsiicdd{iii}ci{ii{d}}ciii{ddd{i}}cdddc{iii{d}}ciii{dd{i}}cd{d}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddd{i}ccddd{i}c{{d{{i}}}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cdddcddd{i}ccddd{i}c{{d{{i}}}}cd{dd{i}}cdcii{ii{d}}c{iiii{iii}}cdd{d}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cddd{i}ciiicddd{d}cd{{ii}}c{d{{i}}}cdsiiic{ii}cdddcd{i}cii{ii{d}}ci{iiii}c{d{{i}}}c{ddd{i}}cd{dd{ii}}ciii{dd{i}}ciicdddcddd{d}ci{iii{d}}ciii{dd{i}}cd{d}cdddci{iii{d}}ciii{dd{i}}cdddd{d}cddd{i}ccddd{i}c{{d{{i}}}}ciii{dd{i}}cdddd{d}cddddcdd{ii}cd{d}cddd{i}ciiicddd{d}ci{iii{d}}ciii{dd{i}}cd{d}cdddcddd{i}ccddd{i}ci{iii{d}}c

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As long as I didn't mess anything up, this should be the shortest possible without () and printing in loops.

Perl 5, 133 bytes

say"S5s0 10 by t513,
T5sh02are 10, 4.
So if210 on t513,
Then 4213 sh0."=~s/\d/(ells,seash," she sells ",ore,"I'm sure","he ")[$&]/ger

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Knight, 138 bytes

O+S=q+=sS=v" shells"4F" se"=tSvT0"sea"F2"S"+" by"+=p" the seashore,
The"+v+s+" are"+t+=u" I'm sure.
So if"+q+S" onn"3Fp+GuF9+s+Gp 4 9+v"."

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Fiiiiiiiine, I'll explain it

Attempt at explaining it

Sorry, the indentation is a little inconsistent...

OUTPUT
 +
  # substitute " s" with "S" for initial capitalization
  SUBSTITUTE(
   = _she_sells_seashells
    + = _she_sells
     # inject " se" into "shells"
     # to get "she sells"
     SUBSTITUTE(
       = _shells " shells"
       4
       0
       " se"
      )
    # inject "sea" to get " seashells"
    = _seashells
       SUBSTITUTE(
        _shells
        1
        0
        "sea"
       )
    )
    0
    2
    "S"
   )
  + " by"
  + = _the_seashore_the " the seashore,\nThe"
  + _shells
  + _she_sells
  + " are"
  + _seashells
  + = _im_sure_so_if "  I'm sure.\nSo if"
  + _she_sells_seashells
  # shorter to inject than concatenate
  # to get " on the seashore,\nThen"
  + SUBSTITUTE(
     " onn"
     3
     0
     _the_seashore_the
    )
  # Extract "I'm sure"
  + GET(
     _im_sure_so_if
     0
     9
    )
  + _she_sells
  # " seashore"
  + GET(
     _the_seashore_the
     4
     9
    )
  + _shells
  : "."

Deadfish~, 1313 1234 bytes

iiissiic{ii}icdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}ddsddc{ii}iiic{{d}i}ic{dd}dsdddddc{d}ddcdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{{ii}}dc{ddd}ddddcdsiiic{ii}cdddc{{d}iii}ic{dd}s{ddd}ic{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}dcdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{{iii}ii}ic{ii}dddc{d}dddc{{d}iii}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{{d}iii}dc{d}ddc{iiii}ic{ddd}ddddc{{i}ddd}c{{d}ii}iiic{dd}s{ddd}iciicdddc{d}dddc{{ii}}ic{{ii}ii}cdsiic{iii}ddc{{d}ii}ic{{iii}iii}dcdddc{{d}iii}c{dd}s{ddd}ic{d}dcdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}ds{d}cdc{{d}ii}iic{dd}dsdddddc{d}ddcdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{{ii}}dc{ddd}ddddcdsiiic{ii}cdddc{i}dc{{d}ii}iic{iiii}ic{ddd}ddddc{{i}ddd}c{{d}ii}iiic{dd}s{ddd}iciicdddc{d}dddc{{d}iii}ic{dd}s{ddd}ic{d}dcdddc{{d}iii}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{{d}iii}ic{dd}s{ddd}ic{d}dcdddc{i}dddcc{i}dddc{{d}iii}ic

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Deadfish, the ungolfing language.

Deadfish~, 1269 bytes

iiissiic{ii}icdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}ddsddc{ii}iiic{ddddddddd}ic{dd}dsdddddc{d}ddcdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{{ii}}dc{ddd}ddddcdsiiic{ii}cdddc{ddddddd}ic{dd}s{ddd}ic{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}dcdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{{iii}ii}ic{ii}dddc{d}dddc{ddddddd}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{ddddddd}dc{d}ddc{iiii}ic{ddd}ddddc{iiiiiii}c{dddddddd}iiic{dd}s{ddd}iciicdddc{d}dddc{{ii}}ic{{ii}ii}cdsiic{iii}ddc{dddddddd}ic{{iii}iii}dcdddc{ddddddd}c{dd}s{ddd}ic{d}dcdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dcdddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}ds{d}cdc{dddddddd}iic{dd}dsdddddc{d}ddcdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{{ii}}dc{ddd}ddddcdsiiic{ii}cdddc{i}dc{dddddddd}iic{iiii}ic{ddd}ddddc{iiiiiii}c{dddddddd}iiic{dd}s{ddd}iciicdddc{d}dddc{ddddddd}ic{dd}s{ddd}ic{d}dcdddc{ddddddd}ic{dd}s{ddd}ic{d}ddddc{i}dddcc{i}dddc{{iiii}iii}dc{dd}s{ddd}ic{d}ddddcddddc{ii}ddc{d}dc{i}dddciiic{d}dddc{ddddddd}ic{dd}s{ddd}ic{d}dcdddc{i}dddcc{i}dddc{ddddddd}ic

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JavaScript: 195 bytes ...?

console.log(`She ${a='sells'} sea${c='shells'} by the sea${e='shore'},
The ${c} she ${a} are sea${c}, ${d="I'm sure"}.
So if she ${a} sea${c} on the sea${e},
Then ${d} she ${a} sea${e} ${c}.`)

Simplest JavaScript answer possible. Simply defines commonly used words. Annoyingly, compression made it longer

Husk, 77 bytes

ΓJx'p¨½ΞĿ∫Ẏj5pż¦Sβyεḟ?ṡ,ḣŀḢ¦ṠεΞĿ∫äeŻaŀ`◄,İ'm ṡ→.¶ȮSÏf£2₅Φεḟ?ṡ,ḣnİ'm ṡ→⁸ρΘŀḢ¦.

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Better than Bubblegum!

Husk's string compression is pretty good for this task, a straightforward compressed string would be 82 bytes long. Here, we apply just a single substitution to make it shorter. The compressed string we use here is

he sells seashpSpells by the seashore,
The shells she sells are seashells, I'm sure.
So if spells on the seashore,
Then I'm sure spore shells.

The structure of the code is:

ΓJx'p¨...
     ¨...  The compressed string
  x'p      split on occurrences of the character 'p'
ΓJ         and joined by interleaving the first substring between the rest

What this does in practice is replacing the character 'p' with "he sells seash" everywhere in the rest of the string. This substitution was the one that generated the shortest compressed string among the ones I've tried; multiple substitutions are possible but they didn't seem worth the longer payload. The letter 'p' was chosen because it generates words like "spell" and "spore" in the text that can be compressed very well by Husk.

Zsh, 128 bytes

<<Z
S${4=${2=he s${1=ells}}${3= seash}$1 }by${5= the$3ore,
The} sh$1 s$2 are$3$1,${6= I'm sure}.
So if s$4on$5n$6 s$2$3ore sh$1.

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Python 3, ^{166 \$\cdots\$ 142} 135 bytes

Saved a whopping 13 18 bytes thanks to ovs!!!
Saved 4 bytes thanks to user253751!!!
Saved a byte thanks to branboyer!!!
Saved 7 bytes thanks to dingledooper!!!

Note: There's lots of unprintables in the following code so please avert your eyes if you're sensitive to such things! :D

print("Sh
 by
 are
,.\nSo if
 on
n
ore.".translate("| seash|ells|ore,\nThe| sh|e s| I'm sure| the".split("|")))

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Based on Netråm's C# answer.

Scala, 155 bytes

printf("S%2$s by%1$s shells%3$sare seashells, I'm sure.\nSo if s%2$s on%1$sn I'm sure%3$sshells."," the seashore,\nThe","he sells seashells"," she sells ")

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PicoLisp, ^{160 157 146 142} 138 bytes

(prin(text"S@1 @2 by the @3,
The @5 s@1 are @2,@4.
So if s@1 @2 on the @3,
Then@4 s@1 @3 @5.""he sells"'seashells'seashore" I'm sure"'shells]

Using the template:

S1 by the 2,
The 5 s1 are 2,4.
So if s1 on the 3,
Then4 s1 3 5.

Where 1 is "he sells", 2 is "seashells", 3 is "seashore", and 4 is " I'm sure"

Ideone It!

{ ^{_{14 imaginary interweb points for guessing which syntax highlighting I'm using without peeking}} }

Common Lisp, 160 bytes

(format t"S~a~a by the ~a,~%The shells s~3:*~Aare ~a, I'm sure.~%So if s~3:*~A~a on the ~a,~%Then I'm sure s~3:*~a~*~a shells.""he sells ""seashells""seashore")

https://ideone.com/n7zVUm

PowerShell, 134 133 132 bytes

-1 byte thanks @Titus

see similar by Nail and Arnauld

$d='S0by36h461 are54,2.
So if60on3n2615ore6h4.'
"154 #he64# I'm6ure# the5ore,
The#ells#6eash# s"-split'#'|%{$d=$d-replace$i++,$_}
$d

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Python 2, 175 158 bytes

thanks @branboyer for the edit!

print"S{0}ells by{1} shells she sells are seashells, I'm sure.\nSo if s{0}ells on{1}n I'm sure s{0}ore shells.".format("he sells seash"," the seashore,\nThe")

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Java 8, 169 160 bytes

v->"".format("S%sells by%s shells she sells are seashells, I'm sure.\nSo if s%1$sells on%2$sn I'm sure s%1$sore shells.","he sells seash"," the seashore,\nThe")

Port of @IgbyLargeman's C# answer, so make sure to upvote him as well!
-9 bytes thanks to @branboyer.

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(Simply returning as is would be 174 bytes.)

Explanation:

v->           // Method with empty unused parameter and String return-type
  "".format(  //  Return the following formatted string:
     "S%sells by%s
      shells she sells are seashells, I'm sure.\n
     So if s%1$sells on%2$s
     n I'm sure s%1$sore shells.",
    "he sells seash",
              //   Where the first `%s` and all `%1$s` are replaced with "he sells seash"
    " the seashore,\nThe")
              //   and the second `%s` and all `%2$s` are replaced with " the seashore,\n
                                                                         The"

PHP, 133 126 bytes

<?=strtr("S2 s101 by t203,\nT2414e s1 are01,5.\nSo if4e s101 on t203,\nT2n54e s10341.",[" seash",ells,he,ore," sh"," I'm sure"]);

Try it online. Use PHP < 7.2 to avoid warnings

JavaScript (V8), 287 bytes

Trying a different way of compressing it, in a different language. Feedback is appreciated!

function s(){a="kdg by the dj,/nThe g care dg, I'm sure./nSo if cdg on the dj,/nThen I'm sure cdj g.".replace(/c|k/g,function(match){return(match=="c")?"she sells ":"She sells ";});c=a.replace(/d|g/g,function(match){return(match=="d")?"sea":"shells";});return (c.replace(/j/g,"shore"));}

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V (vim), 110 bytes

iS230 by41h012 are30,5.
So if1230 on4n5123ore1h0.Í5/ I'm1ure
Í4/ the3ore,\rThe
Í3/1eash
Í2/he10
Í1/ s
Í0/ells

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Port of @Neil's retina answer to V (vim)

Japt, ^{108 105 104 96 93} 92 bytes

`S230 by41h012 ‡e30,d.
So if1230 4˜123Že1h0.`dd` I'm1¨e`4` e3Že,
T”`3`1Á`2`”10`1" s"0`e¥s

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Port of the retina answer to Japt.

Explanation

S230 by41h012 ‡e30,d.
So if1230 4˜123Že1h0.

Decompress into the string literal:

S230 by41h012 are30,d.
So if1230 on4nd123ore1h0.

Then we do replacements with:

d
 d` I'm1¨e`     // "d" with " I'm1ure"
 4` e3Že,\nT”`  // "4" with " the3ore,\nThe"
 3`1Á`          // "3" with "1eash"
 2`”10`         // "2" with "he10"
 1" s"          // "1" with " s"
 0`e¥s          // "0" with "ells"

PowerShell, 158 bytes

Because if bash counts then why not...

$1="seash"
$5="ells"
$4="he s$5 "
$6=" I'm sure"
$2="$1$5"
$3="$1`ore"
"S$4$2 by the $3,`nThe $5 s$4`are $2,$6.`nSo if s$4$2 on the $3,`nThen$6 s$4$3 $5"

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FEU, 112 111 bytes

-1 byte thanks to Neil

a/S256 by43h632 are56,1.\nSo if3256 on4n1325ore3h6.
m/1/ I'm3ure/2/he36/3/ s/4/ the5ore,\nThe/5/ seash/6/ells/g

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Port of JavaScript answer.

zlib, 84 bytes

Hex dump:

00000000: 78 9c 0b ce 48 55 28 4e cd c9 29 06 92 89 c5 19  x...HU(N..).....
00000010: 60 56 52 a5 42 09 58 18 28 90 5f 94 aa c3 15 02  `VR.B.X.(._.....
00000020: e2 41 e4 8a e1 ea 13 8b 52 11 7a 74 14 3c d5 73  .A......R.zt.<.s
00000030: 15 8a 4b 8b 52 f5 b8 82 f3 15 32 d3 90 14 22 0c  ..K.R.....2...".
00000040: ce cf c3 34 38 0f ae 13 5d 4b 7e 11 cc 56 3d 00  ...48...]K~..V=.
00000050: 3c 70 3a e6                                      <p:.

Raw hex:

789c0bce4855284ecdc929069289c519605652a54209581828905f94aac31502e241e48ae1ea138b52117a74143cd573158a4b8b52f5b882f31532d39014220ccecfc334380fae135d4b7e11cc563d003c703ae6

Or generate this file yourself:

openssl zlib < seashells.txt > seashells.zlib

You can decompress this file execute this program with:

openssl zlib -d < seashells.zlib

or, presumably, any of the answers here.

Python, 179 176 bytes

print"She sells seashells by the seashore,\nThe shells she sells are seashells, I'm sure.\nSo if she sells seashells on the seashore,\nThen I'm sure she sells seashore shells."

Trivial algorithm, but nobody’s done it yet. I’m working on a better answer in a different language, but I don’t know if it should be posted in another answer or in this one. Shortening advice accepted, but I’m perfectly fine with just hard-coding it for now.

ink, 129 130 bytes

-(i){|The shells|So if|Then I'm sure} {S|s}he sells {|are |}sea{|||shore }shells{& {on|by} the seashore,|{, I'm sure.|.->END}}
->i

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• +1 byte: A period was missing from the end of a line

Explanation

// Thanks to all the new whitespace, this ungolfed code doesn't quite work as is
-(i)                              // A label
{|The shells|So if|Then I'm sure} // A sequence - the first time we get here we print nothing,
                                  // the second time we print "The shells",
                                  // the third time we print "So if", etc
{S|s}                             // Sequences keep outputting their last value once it's been reached
he sells                          // Plain text just gets printed
{|are |}
sea{|||shore }shells
{& {on|by} the seashore,|{, I'm sure.|.->END}}
                                  // {& marks a repeating sequence.
                                  // Instead of getting stuck on its last value, this it starts over.
                                  // On the fourth pass we hit ->END and terminate the program
->i                               // On the other passes, we get here and go back to the start

Bash, 156 bytes

i=" I'm sure"
h=" she"
k=lls
l=" seashe"$k
s=" seashore"
e=" se"$k
printf "She$e$l by the$s,
The$h$k$h$e are$l,$i.
So if$h$e$l on the$s,
Then$i$h$e$s$h$k."

An expert can probably save some more here.

Bubblegum, 79 78 bytes

^{removed trailing newline for -1 byte}
The text in the tongue twister has many simple patterns. There are enough of them that even "normal" languages can save bytes. However, the only thing DEFLATE does is compressing simple patterns with little overhead...

00000000: 0bce 4855 284e cdc9 2906 9289 c519 6056  ..HU(N..).....`V
00000010: 52a5 4209 5018 2c90 5f94 aac3 1502 e441  R.B.P.,._......A
00000020: e58a e1ea 138b 5211 7a74 143c d573 158a  ......R.zt.<.s..
00000030: 4b8b 52f5 b882 f315 32d3 800a 310d cecf  K.R.....2...1...
00000040: c334 380f ae13 a105 a602 66ab 1e00       .48.......f...

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After "writing" this answer, I finally decided to find a way to view the internal representation details of the string (to be specific, information on what characters are printed literally and what substrings are compressed as repetitions via LZ77). I modified kunzip (which already had some useful logging capabilities) to log only the information I want to see. This is the result:

output 83 S
output 104 h
output 101 e
output 32  
output 115 s
output 101 e
output 108 l
output 108 l
output 115 s
len: 3 dist: 6 ( se)
output 97 a
output 115 s
output 104 h
len: 5 dist: 10 (ells )
output 98 b
output 121 y
output 32  
output 116 t
len: 3 dist: 23 (he )
len: 5 dist: 17 (seash)
output 111 o
output 114 r
output 101 e
output 44 ,
output 10
output 84 T
len: 3 dist: 14 (he )
len: 7 dist: 28 (shells )
output 115 s
len: 9 dist: 48 (he sells )
output 97 a
output 114 r
output 101 e
len: 10 dist: 52 ( seashells)
output 44 ,
output 32  
output 73 I
output 39 '
output 109 m
output 32  
output 115 s
output 117 u
output 114 r
output 101 e
output 46 .
output 10
output 83 S
output 111 o
output 32  
output 105 i
output 102 f
len: 3 dist: 41 ( sh)
len: 18 dist: 89 (e sells seashells )
output 111 o
output 110 n
len: 18 dist: 89 ( the seashore,
The)
output 110 n
len: 9 dist: 58 ( I'm sure)
len: 10 dist: 51 ( she sells)
len: 8 dist: 34 ( seashor)
len: 8 dist: 118 (e shells)
output 46 .

I think might will be useful for "normal" language answers.

C# (Visual C# Interactive Compiler), 154 bytes

Write(@"S{0}sea{1} by the {2},
The {1} s{0}are sea{1}, I'm sure.
So if s{0}sea{1} on the {2},
Then I'm sure s{0}{2} {1}.","he sells ","shells","seashore")

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JavaScript (ES6),  142  141 bytes

Saved 1 byte thanks to @Neil

_=>`246 / I'm5ure/he56/ the4ore,
The/5eash/ s/ells`.split`/`.reduce((s,p,i)=>s.split(i).join(p),`S0by35h652 are46,1.
So if50on3n1524ore5h6.`)

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Compression steps

"ells" -> "6" (9 occurences, saved: 22 bytes)
She s6 seash6 by the seashore,
The sh6 she s6 are seash6, I'm sure.
So if she s6 seash6 on the seashore,
Then I'm sure she s6 seashore sh6.

" s" -> "5" (17 occurences, saved: 14 bytes)
She565eash6 by the5eashore,
The5h65he56 are5eash6, I'm5ure.
So if5he565eash6 on the5eashore,
Then I'm5ure5he565eashore5h6.

"5eash" -> "4" (6 occurences, saved: 18 bytes)
She5646 by the4ore,
The5h65he56 are46, I'm5ure.
So if5he5646 on the4ore,
Then I'm5ure5he564ore5h6.

" the4ore,\nThe" -> "3" (2 occurences, saved: 10 bytes)
She5646 by35h65he56 are46, I'm5ure.
So if5he5646 on3n I'm5ure5he564ore5h6.

"he56" -> "2" (4 occurences, saved: 7 bytes)
S246 by35h652 are46, I'm5ure.
So if5246 on3n I'm5ure524ore5h6.

" I'm5ure" -> "1" (2 occurences, saved: 5 bytes)
S246 by35h652 are46,1.
So if5246 on3n1524ore5h6.

"246 " -> "0" (2 occurences, saved: 1 byte)
S0by35h652 are46,1.
So if50on3n1524ore5h6.

Retina 0.8.2, 106 103 bytes

S230 by41h012 are30,5.¶So if1230 on4n5123ore1h0.
5
 I'm1ure
4
 the3ore,¶The
3
1eash
2
he10
1
 s
0
ells

Try it online! Explanation: In Retina, a substitution only makes sense if it's long enough l for the number n of repetitions. The substitution saves n(l-1) bytes in the compressed text but costs l+3 bytes in the replacement stages. This gives the minimum length required to be useful as follows:

• 2 repetitions: length > 5
• 3 repetitions: length > 3
• 4 repetitions: length > 3
• 5 repetitions: length > 2
• 6+ repetitions: length > 1

Edit: As @Arnauld pointed out, you can count repetitions from the substitution entries as well. This means that although there were only 5 repetitions of space+s in my previous encoded text, there are also 3 repetitions in the substitutions, thus allowing me to save 3 bytes overall. (@Arnauld himself had only spotted 2 of the 3 repetitions.)

Stax, 83 bytes

ÇσGª☺Uë\╠╤↑╡V╦⌐∞┐ü9B@♦1ÖFò╫◄«vâ¬♂ƒQn→↨¥p▬♂♥å↓hb½Z╫Éà½♣µ\╩N>.▄┌bæ¼@3▲¡w8╣▐ù☼ve/h{≈ï░

Run and debug it

How?

Unpacked:

`...`'!/|J
`...`      - compressed string "She sells..." with '!' where newlines should be
     '!    - a '!' character
       /   - split the compressed string at '!' characters
        |J - join with newline characters

Maybe making a manual compression using the words will be shorter?

05AB1E, 91 86 bytes

“SheÌç1€‹€€0,
The¬•s‚ÎÌ瀙1,2.
So€¬‚ÎÌç1€‰€€0,
Then2‚ÎÌç0¬•s.“2Ý…½š¬•s#'í™ì“I'mˆ•“ª1ú:

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Explanation:

“SheÌç1€‹€€0,
The¬•s‚ÎÌ瀙1,2.
So€¬‚ÎÌç1€‰€€0,
Then2‚ÎÌç0¬•s.“  # Push dictionary string "She sells1 by the0,
                 #                         The shells she sells are1,2.
                 #                         So if she sells1 on the0,
                 #                         Then2 she sells0 shells."
2Ý               # Push list [0,1,2]
…½š¬•s           # Push dictionary string "shore shells"
      #          # Split it on spaces: ["shore","shells"]
       'í™ì     '# Prepend dictionary string "sea" in front of both:
                 #  ["seashore","seashells"]
“I'mˆ•“ª        '# Append dictionary string "I'm sure" to this list:
                 #  ["seashore","seashells","I'm sure"]
1ú               # Pad each string with a leading space:
                 #  [" seashore"," seashells"," I'm sure"]
:                # Replace all [0,1,2] with [" seashore"," seashells"," I'm sure"]
                 # (after which the result is output implicitly)

See this 05AB1E tips of mine (section How to use the dictionary?) to understand how the dictionary strings work.

///, 115 bytes

Port of Neil's answer + accepting nph's suggestion.

/3/ I'm4ure//2/4eash//1/he40//0/ells//4/ s/S120 by the2ore,
The4h041 are20,3.
So if4120 on the2ore,
Then3412ore4h0.

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C# (Visual C# Interactive Compiler), 171 bytes

string a=" she sells ",b="shells",c="seashore",d=a+"sea"+b;Write(@$"She{d[4..]} by the {c},
The {b}{a}are sea{b}, I'm sure.
So if{d} on the {c},
Then I'm sure{a}{c} {b}.")

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The @ allows for line breaks inside string literals.