“I wanted to try water skiing but I couldn’t find a sloping lake”

Water skiing doesn’t require a slope (like a ski slope), but there are jokes. “Joe Kowalski hasn’t used his new water-skis yet; he can’t find a sloping lake: was printed in The Age (Melbourne, Victoria, Australia) on July 30, 1974.
 
“I recently gave up trying to waterski. I couldn’t find a sloping lake” was posted on Reddit—3amjokes on July 31, 2020. Authorship of the water skiing joke is unknown.
 
     
Wikipedia: Water skiing 
Water skiing (also waterskiing or water-skiing) is a surface water sport in which an individual is pulled behind a boat or a cable ski installation over a body of water, skimming the surface on two skis or one ski.
 
Newspapers.com
30 July 1974, The Age (Melbourne, Victoria, Australia), “Ebenezer” by Peter Smark, pg. 2, col. 9:
Joe Kowalski hasn’t used his new water-skis yet; he can’t find a sloping lake. 
 
1 November 1983, The Spectator (London, UK), “Sad Irish joke” by Richard West, pg. 20, col. 2:
A few years ago, one used to see roneoed sheets of ‘Irish jokes’...like the one about the Irishman who bought himself some water-skis then couldn’t find a sloping lake…
           
Google Groups: rec.humor
Some Q & A jokes for kids
David Bridges
8/7/97
(...)
Did you hear about the idiot who wanted to water ski?
He gave up the idea because he couldn’t find a sloping lake.
     
Twitter
Brian Miles
@twittoyoo
I want to go water-skiing…does anybody know where I can find a sloping lake ?
5:46 PM · Apr 20, 2009·Twitter Web Client
 
Twitter
SpiderSam
@Sarkysammo
I always wanted to go water skiing, but could never find a sloping lake!!
2:07 AM · Mar 30, 2012·Tweetbot for iOS
 
Google Books
Horrid Henry’s Hilariously Horrid Joke Book
By Francesca Simon
London, UK: Orion Children’s Books
2014
Pg. ?:
Why didn’t Beefy Bert enjoy his water skiing holiday?
He couldn’t find a sloping lake.
         
Twitter
Martin Holmes
@martin3007
I’m thinking of trying water skiing. But I just cannot find a sloping lake anywhere.
6:35 PM · Nov 4, 2014·Twitter Web Client
 
Twitter
Stephen Burns
@Sweetcorn2
Replying to @tonyblackburn
I wanted to try water skiing but couldn’t find a sloping lake.
8:52 AM · Apr 26, 2018·Twitter for Android
 
Twitter
Badly Packed Kebab
@BulldogEvo3
Replying to @ScouseDave4
I wanted to take up skiing but couldn’t find a sloping lake
10:22 AM · Apr 3, 2020·Twitter for Android
 
Reddit—3amjokes
Posted by u/DiamondUnicorn July 31, 2020
I recently gave up trying to waterski.
I couldn’t find a sloping lake.