“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A eel version is:
   
“An eel walks into a bar. The bartender says, ‘Back for more, eh (moray)?’”
 
The joke has been cited in print since at least 2015 and has been printed on an image.
     
“What did the Italian say after he ate an eel pizza?”/“That’s a moray” is another moray eel joke.
 
 
Wikipedia: Moray eel
Moray eels, or Muraenidae, are a cosmopolitan family of eels. The approximately 200 species in 15 genera are almost exclusively marine, but several species are regularly seen in brackish water, and a very few are found in fresh water.
   
Thrillist 
38 NEW “ANIMAL WALKS INTO A BAR” JOKES, BECAUSE PUNS ARE AWESOME
By DAVID BLEND
Published On 10/26/2015
@daveblend
(...)
An eel walks into a bar. Bartender says, “Back for more, ay?”
   
Twitter
Pete‏
@P_Geros
An eel walks into a bar. Bartender says, “Back for more, ay?”
7:25 AM - 12 Nov 2015
 
Twitter
❤‏
@i_sing_my_heart
#NationalTellAJokeDay
An eel walks into a Scottish bar. Bartender says, “Back for more, ay?”
9:54 AM - 16 Aug 2016
 
Twitter
Alex Jeffries‏
@AlexJeffries1
So an eel walks into a bar and the bartender says… So you’ve come back for mor… ay. 😂😂
6:40 AM - 8 Sep 2016
 
Twitter
Brad Moran‏
@badbrad1727
Replying to @rhettandlink
an eel walks into a bar, bar tender says back for more eh? (moray)
7:05 PM - 11 Dec 2016