“Seventy-eight percent of the earth is covered with water; the other 22 percent is covered with mortgages” is a joke illustrating that most of the earth is actually under water—and the part that is land is financially under water. The joke has been printed in many newspapers since 1980 and is of unknown authorship.
 
 
15 April 1980, Seattle (WA) Times, “the Old Ball Game” by Walt Evans,” pg. B2, col. 1:
We will leave you with this prime-rate thought: 78 per cent of the earth is covered with water. The other 22 per cent is covered with mortgages.
 
3 July 1980, Chicago (IL) Tribune, pg. A1:
Seventy-eight per cent of the earth is covered with water; the other 22 per cent is covered with mortgages.

15 July 1980, The Gazette(Cedar Rapids, IA), pg. 1, col. 1:
Today’s Chuckle
Seventy-eight percent of the earth is covered with water. The other twenty-two percent is covered with mortgages.
 
Google News Archive
1 March 1981, Daytona Beach (FL) Sunday News-Journal,  “Quips and Quotes,” Family Weekly magazine, pg. 19, col. 2:
Here’s something of (high) interest: Seventy-eight percent of the earth is covered with water; the other 22 percent is covered with mortgages.—M. R.
 
Google News Archive
10 February 1982, Spartanburg (SC) Herald, “The Stroller” by Seymour Rosenberg, pg. 1, col. 1:
Seventy-eight percent of the earth is covered with water. The other 22 percent is covered with mortgages.
 
Google Books
Aussie Gags
By John Blackman
Sydney: Pan Macmillan
1998
Pg. ?:
Seventy-eight per cent of the Earth is covered with water. The remaining twenty-two per cent is covered with mortgages.
 
Google Books
Jolly Jokes for the Young at Heart
By Bob Phillips
Eugene, OR: Harvest House Publishers
2010
Pg. 96:
Mortgage
Seventy percent of the earth is covered with water, and the other thirty percent is covered with mortgages.