“A guy walks into a bar...” is a typical form of what has been called the “bar joke.” A fish version is:

“A fish walks into a bar. The bartender says, ‘What do you want?’ The fish croaks, ‘Water.’”

The joke has been cited in print since at least 1997. “A goldfish walks into a bar...” is a similar joke that has been cited in print since at least 2006.


Google Groups: alt.sysadmin.recovery
Why ORA is that cool.
reality.
6/1/96
(...)
A fish walks into a bar, completely skewing all laws of probablility in the universe which, subsequently, implodes.

Google Groups: microsoft.public.excel.misc
Running XLS sheets as standalone programmes
Timothy Pratt
4/2/97
(...)
A fish walks into a bar and the bartender says:
‘What do you want’?
The fish crocks ‘water’.

Google Groups: alt.lifestyle.furry
Laugh, damn you!
Kai Robinson
1/6/99
(...)
A fish walks into a bar and the bartender says: “What do you want?”
The fish says “...*gasp*...water!”

Google Books
Man Walks into a Bar:
Over 6,000 of the Most Hilarious Jokes, Funniest Insults, and Gut-Busting One-Liners
By Stephen Arnott and Mike Haskins
Berkeley, CA: Ulysses Press
2007
Pg. 114:
A fish walks into a bar. The bartender says, “What do you want?” The fish croaks, “Water.”

Google Books
The Mammoth Book of Really Silly Jokes
By Geoff Tibballs
London: Constable & Robinson Ltd.
2011
Pg. 103:
A fish walks into a bar. The bartender asks, “What would you like?”
Holding its neck, the fish gasps, “Water.”

Twitter
Amy Rey
‏@amyrey_uk
A fish walks into a bar. The bartender asks, “What’ll you have?”
The fish croaks, “Water!”
#jokes
2:40 AM - 8 Jan 2016