Hi Chris,
I play around with the water once in a while, and here's what I can say for sure. When you change something in their enclosure, they become more active and investigate it. Over time, however, they get used to the new object and take it's presence for granted.
The normal water dish that I use is 6" x 7" (15 x 18cm). Sometimes I add one that is 13" x 9" (33 x 23cm). The larger water dish is about twice as deep, and I usually add small fish to the larger water dish but not to the smaller one. I enjoy seeing the snakes swim around in the larger dish to chase the fish. They get more exercise, but a day or two after the fish are gone, they don't just swim around for exercise.
These are just my observations, but my sample size for different snakes' behavior isn't large enough to say anything definitively with confidence. Maybe other people have different experiences, too? Just changing the water dish stimulates them, so there aren't any negative consequences.
Rick
P.S. When I want them to swim, I put them in the bathtub, but don't tell my wife
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