Polo is played on horses; water polo is not. A popular joke is:
 
“I tried playing water polo, but my horse drowned.”
 
The joke has been cited in print since at least 1968.
   
 
18 May 1968, Delaware County Daily Times (Chester, PA), Earl Wilson syndicated entertainment column, pg. 23, col. 8:
EARL’S PEARLS: Ben Benson says he used to play water polo, but had to give it up: “My horse was drowned.”
   
Google News Archive
22 January 1976, Milwaukee (WI)

, “Ford Aide Said Jokes Hurt Bid,” Accent on the News, pg. 2, col. 1:
An example is the one that says (U.S. President Gerald—ed.) Ford was playing water polo but had to give it up because the horse drowned.
   
Google Books
June 1978, Boys’ Life, “Think & Grin,” pg. 80, col. 2:
Nit: I used to play water polo.
Wit: Why did you stop?
Nit: My horse drowned, — Neal Jodeit, Minneapolis, Minn.
   
Google Books
Doc Blakely’s Handbook of Wit and Pungent Humor
By James Blakely
Houston, TX: Rich Publishing Company
1980
Pg. 153:
Hear about the guy who gave up water polo? His horse drowned.
 
Google News Archive
26 September 1988, Pittsburgh (PA) Post-Gazette, “Olympic fever spreads to poll” by Peter Leo, pg. 8, col. 1:
He became despondent when his horse drowned during a water polo match.
 
Twitter
Charlie Kidsneir
‏@lobcitypolo
I tried playing water polo today but my horse drowned.
8:22 PM - 30 Mar 2015