That's really a cool idea! In my experience (20 years keeping quite a few rattlebrats) they do like water. I took in some unwanted captives that had been kept in
filthy cages by people who lost interest (<I'm being charitable) & they stunk, so I did bathe them in my bathtub. They seemed to like it, especially to get clean.
The water was not deep though. While I agree that (supervised) swimming is great exercise for captive snakes (-most kinds), I didn't have a problem with my
rattlesnakes becoming obese either, maybe because their cages were fairly large.
BTW, I personally swim laps for fitness, and it's hard to find a better exercise to stay trim that's both aerobic & not too hard on the body.