Of the 4 species I have, all only occasionally rest in their water tanks. They seem to do this most when they expect to be fed or at times only to soak for an hour or two to relax. I think that they would only "swim" to search for food or to access another basking site (or to escape predators if required). If their water area was much larger then perhaps they would show a more natural willingness to include this in their semi-aquatic nature. Michael E.