What I would do, and I'm not saying you have to do it, especially if the water is over 85F, is to remove the large soaking bowl, tub, or whatever it is, and replace it with a ceramic bowl about 8 " in diameter and 1 1/2 inches tall. If it's made of ceramic, he can't step on the edge and flip it over, and he can't get in it to sleep.
IMHO it's not logical that this is natural behavior. Sleeping in water puts them at high risk of being eaten by a snake or a caiman/alligator. But I have heard that in the wild they do drop into water from high up in the canopy. Maybe to get away from a hawk or large tree snake.