“It’s so dry, the catfish are carrying canteens” is a version of a very old joke. In 1902, it was said that Southern California was so dry the coyotes were carrying canteens to cross Death Valley. In 1928, Nevada trout were said to be carrying canteens. In 1941, Arizona fish were said to be carrying canteens. In 1957, Texas cows were said to be carrying canteens.
 
The catfish version of carrying canteens has been cited in print since at least 1993.
 
 
Google Books
Historical and Biographical Record of Southern California:
Containing a history of southern California from its earliest settlement to the opening year of the twentieth century ; also containing biographies of well-known citizens of the past and present
By J. M. Guinn
Chicago, IL: Chapman Pub. Co.
1902
Pg. 199, col. 2:
It was mesa or tableland that had never been cultivated, and so dry that one old timer said he had seen “the coyotes carrying canteens when they crossed it.”
 
22 February 1928, Reno (NV) Evening Gazette, pg. 8, cols. 2-3:
Trout Carrying Canteens
Immortal Toads and Blind
Fish All Mixed up in Tale
 
Texas Curio Rouses Memory of Old-Timers Who Tell
Of Nevada Winders; Prison Footprints
And Live Toad Recalled
(...)
In times of plenty of water the fish swam merrily in schools and also prepared fir the dry season. They were provided with canteens the old timers said and found a nice cool spot under the desert sands to reside during the dry season only to come to life again when the spring floods from the mountains furnished water to refill the canteens.
 
Google News Archive
17 January 1941, Prescott (AZ) Evening Courier, pg. 6, col. 3:
Quite a difference from last year when all the fish were carrying canteens and the frogs were preparing to emigrate overland to the Colorado river.
   
22 January 1948, Van Nuys (CA) News, “News-Graphs” by Walter Mendenhall, pg. 1, col. 1:
THIRSTY FISH
Anent the dry season, here is one that caps the climax: Lompoc publisher we met at Coronado convention last week said there are a few steelhead coming up the Santa Ynez river, but that they are all carrying canteens.
 
25 September 1955, Omaha (NE) World Herald, “Rich Valleys Where Two Platte Rivers Come Together” by Robert Agee, Magazine, pg. 26, cols. 2-3:
It was once a common sight to see those canals bone-dry and a jackrabbit carrying a canteen down the banks.
 
Google News Archive
30 May 1957, Bonham (TX) Daily Favorite, pg. 6, col. 1:
The Nation’s Driest Rancher…
Cows Carrying Canteens West of Pecos
MADISONVILLE, Tex. (UP)—A West Texas rancher who says his land is so dry “my cows are carrying canteens” is the winner of Madisonville Cattlemen’s Association “most drought-stricken rancher in the United States” contest.

Buck Jackson, whose ranch is located west of Pecos, Tex., near the New Mexico border, said the “drifting sands have dammed up the Pecos River, and the government is hauling out the river bed and giving it away as foreign aid.
 
“I’m still hoping for rain,” Jackson said. “My children saw it rain once.”
 
3 October 1966, Dallas (TX) Morning News, “MaybeOld 1956 Wasn’t So Great” by Kent Biffle, pg. D3, col. 8:
DALLAS PEOPLE weretelling each other thatthe jackrabbits were carrying canteens.
 
2 January 1977, Seattle (WA) Daily Times, pg. B10, col. 2:
Portland was so dry that…
PORTLAND—(AP)—How dry was it?
 
It was so dry, even the camels were carrying canteens.
   
Google Books
Nashville Jamboree: a full-length musical
By Jim Stuart, Tim J. Kelly and Mary Stuart
Boston, MA: Baker’s Plays
1979
Pg. 67:
COUSIN BELLE. Hot! I tell you, Aunt Tish, it’s been so hot I seed a coyote chase a jackrabbit down the road and they (Pg. 68—ed.) was walking! [Laughter] And that jackrabbit was carrying a canteen!
     
Google News Archive
23 August 1988, Eugene (OR) Register-Guard, pg. 8A, col. 3:
One old boy stopped in the other day and said, “It’s so dry the cows are giving powdered milk and the ducks are carrying canteens.”
 
Google Books
In the Electric Mist with Confederate Dead
By James Lee Burke
New York, NY: Avon
1994, ©1993
Pg. 18:
“I bet you won’t believe this,” the deputy said, “but it’s been so dry here on occasion that I seen a catfish walking down the levee carrying his own canteen.”
 
More Colorful Texas Sayings Than You Can Shake a Stick At
Texans have unique ways of expressing their feelings. Common as cornbread, old as dirt, funny as all get-out—homespun expressions link modern Texans to our rural and agricultural past, conveying the resolute spirit and plainspoken humor of our heroes and pioneers. Some sayings are instantly familiar because our parents and grandparents quoted them; others parallel the wisdom of biblical proverbs or Poor Richards Almanac.
—Anne Dingus, Texas Monthly, December 1994
Here is a collection of the most geographically relevant expressions by category.
(...)
Dry
So dry the birds are building their nests out of barbed wire.
So dry the Baptists are sprinkling, the Methodists are spitting, and the Catholics are giving rain checks.
So dry the catfish are carrying canteens.
     
8 August 1998, Augusta (GA) Chronicle, pg. 5A, col. 5:
Before the rains came Augusta was so dry that grasshoppers were carrying canteens.