A “paradox” (“pair of docks”) riddle is:
 
Q: What’s a paradox?
A: Two places to park your boat.
 
“A paradox is two places to park boats” has been cited in print since at least 1953.
   
             
Google Books
The Virginia Spectator
1949 (The Google Books date is possibly incorrect.—ed.)
Pg. 38:
A paradox is two places to park boats.
 
23 August 1953, Detroit (MI) Times, “The Cheering Section” conducted by Bruce Patterson, Pictorial Review, pg. 9, col. 1:
A paradox is two places to park boats.
Sundial
     
20 September 1953, Omaha (NE) World-Herald, “The Cheering Section” conducted by Bruce Patterson, Magazine, pg. 23G, col. 1:
A PARADOX is two places to park boats.
—Sundial.
 
2 October 1960, Boston (MA) Sunday Advertiser, “The Cheering Section” conducted by Bruce Patterson, Pictorial Review, pg. 9, col. 4:
Deft-nition: paradox—two places to park boats.
Crimson Bull
 
19 June 1962, Orlando (FL) Sentinel, “Sentinel Laughs,” pg. 4, col. 4:
Percy—“Say, what is a paradox?
Flage—“Well, it’s either a couple of physicians, or two places to park boats.”
 
Google Books
Farm Journal
Volume 86
1962
Pg. 12:
Explained — PERCY — “Say, what is a paradox?”
FLAGE — “Well, it’s either a couple of physicians, or two places to park boats.”
       
Google Books
The Ford Almanac
New York, NY: Golden Press
1964
Pg. 38:
DEFINITION: Paradox: Either a couple of physicians or two places to park a boat.
   
3 May 1965, Garden City (KS) Telegram“Garden Sass,” pg. 1, col. 7:
A paradox, Gus Garden says, is either a couple of physicians or two places to park a boat.
 
Google Groups: rec.autos.sport.info
F1: Hill, Williams talks rumoured to have broken down
Michael Cosgrove
8/31/96
(...)
Paradox: Two doctors or two places to tie up your boat.
 
Twitter
J T. Ramsay
@jtramsay
Replying to @Jack_Beitz
.@jack_beitz @LeeRussakoff Q: What’s a paradox? A: Two places to park your boat.
8:37 AM - 21 Jun 2012
   
Twitter
D-Town
@Kcso_Townsend
What’s a paradox? Two places to park your boat.
5:56 PM - 19 Jul 2012
   
Reddit—Funny
If there are two places to drop off your boat (self.funny)
submitted April 15, 2015 by sportsman958
Would it be a paradox?
   
Reddit—Jokes
Why can’t you have two places to park your boat?
submitted August 22, 2017 by Chuck_Schuldiner
Because that’s a paradox!
 
Reddit—Oneliners
I have two places I could tie off my one boat. It’s quite the paradox.
submitted June 30, 2018 by goldomega