“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A goldfish version is:
 
“A goldfish walks into a bar and looks at the bartender. The bartender asks, ‘What can I get you?’ The goldfish says, ‘Water.’”
   
The joke has been cited in print since at least 2006. “A fish walks into a bar…” is a similar joke that has been cited in print since at least 1997.
           
 
Google Groups: misc.news.internet.discuss
Needs Rimshots
Dave K
3/17/06
(...)
A goldfish walks into a bar and looks at the bartender. The bartender asks, “What can I get you?” The goldfish says, “Water.”
   
Google Groups: alt.life.sucks
Two jokes for you
Doug Donaghue
5/22/06
(...)
A goldfish walks into a bar.
“What can I get you?”, asks the Bartender.
“Water”, replies the goldfish.
 
Twitter
Brian Herman
‏@brianherman
Your daily groan:  A goldfish walks into a bar, The bartender asks, “What can I get you?” The goldfish says, “Water.”
9:04 AM - 10 Oct 2008
   
Google Groups: alt.messianic.yeshua
Re: TRUTH, Pure and Simple
Anon
12/7/08
(...)
A goldfish flops into a bar and looks at the bartender. The bartender asks, “What can I get you?” The goldfish says, “Water.”
 
Google Books
The Best ‘A Man Walks Into A Bar’ Jokes
By Billy Brownless
Crows Nest NSW: Allen & Unwin
2009
Pg. 51:
A goldfish walks into a bar and looks at the bartender. The bartender asks, “What can I get you?” The goldfish says, “Water.”
 
Google Books
The Mammoth Book of One-Liners
By Geoff Tibballs
London: Constable & Robinson Ltd
2012
Pg. ?:
A goldfish flops into a bar. THe bartender says, “What can I get you?” The goldfish gasps: “Water.”