A popular boat riddle is:
 
Q: Where do boats go when they’re sick?
A: To the dock (doc).
 
The riddle has been cited in print since at least 1967.
 
   
30 October 1967, Minneapolis (MN) Star, “Boat Joke,” pg. 5B, col. 2:
Debra asks, “Where do they send boats when they are sick?”
 
The answer is, “To the dock.”
 
9 January 1971, Chicago (IL) Tribune, “Riddle,” sec. 1, pg. 13, col. 6:
Where do they send sick boats?
To the dock.
   
18 March 1978, Battle Creek (MI) Enquirer and News, “Mini Jokes,” The Mini Page, pg. 3, col. 2:
Q. Where do boats go when they’re sick?
A. To the dock!
—Wendy Shepard, Battle Creek
   
5 November 1989, Newsday (Long Island, NY), “Smiles,” Kidsday, pg. 1:
Where do sick boats go? To the dock.
Kevin Starkes, West Hempstead
 
Google Books
Super Giggles:
Knock-Knocks, Jokes, and Tongue-Twisters
New York, NY: Barnes & Noble
2005
Pg. 164:
Where do boats go when they’re sick?
To the dock-
   
Reddit—3amjokes
Where do boats go when they’re sick? self.3amjokes
Submitted October 8, 2014 by GuyWhoNeverComments
 
Reddit—Jokes 
Where do sick boats go?
submitted December 7, 2014 by Blue_sweater_
........to the dock!!
HAHHAHHAHAHHA
 
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
Where do boats go when they are sick?
To the dock.