A “loan shark” riddle is:
 
Q: Where do fish go to borrow money?
A: A loan shark.
 
The riddle has been cited in print since at least 2007.
 
   
Wikipedia: Loan shark
A loan shark is a person or body who offers loans at extremely high interest rates. The term usually refers to illegal activity, but may also refer to predatory lending with extremely high interest rates such as payday or title loans. Loan sharks sometimes enforce repayment by blackmail or threats of violence. Historically, many moneylenders skirted between legal and extralegal activity. In the recent western world, loan sharks have been a feature of the criminal underworld.
   
20 May 2007, The Post-Tribune (Gary, IN), “Prayers Really Do Work,” pg. A4:
“Where can a fish borrow money? From a loan shark.”
 
LisaMullarkey.com (July 15, 2007)
~Just Beachy Jokes!~
(...)
Where does a fish go to borrow money?  A loan shark!
   
Google Groups: Angelique’sSmorgaabord
Crazy Corner
.(JavaScript must be enabled to view this email address)
6/20/08
(...)
To whom do fish go to borrow money ?
The loan shark !
 
Twitter
Ocean City, Maryland
‏@OceanCity
DID YOU KNOW ... that when a fish needs to borrow money it goes to a loan shark?  😊
7:31 AM - 8 Apr 2009
 
18 September 2010, Herald Express,(Torquay, UK), “Jokes,” pg. 18:
TO whom do fish go to borrow money? The loan shark!
 
Google Books
Jokes for Kids:
Jokes, Riddles, & Tongue Twisters That Will Tickle Your Ribs & Make Your Funny Bone Laugh
By Nicole Townsend
Lulu Publishing (Lulu.com)
2014
Pg. ?:
Q: Where can aquatic animals go to borrow money?
A: A ‘loan’ shark!
 
21 August 2014, Evening Chronicle (Newcastle-upon-Tyne, UK), “Joke TIme,” pg. 54:
Where does a fish go to borrow money? A loan shark!
 
Twitter
Crap Jokes
‏@TheCrapJoker
Q) Where do fish go to borrow money?
A) A Loan Shark.
1:30 PM - 19 Jun 2016