Actually I let my snakes swim once in a while and some of my milks and most of my kings seemed to enjoy it swimming around gently and not freaking out. You'd have to make sure that the water's not too warm, I'd err on the side of cold. I let them swim as a form of exercise.
Snakes are pretty clean animals but in my experience, I've seen snakes which have just pooped and gone over it, smearing themselves. In fact I just found one of my albino kings with some poop on her face. This comes from captive snakes being housed in a limited area where there is a chance they may crawl over their poop. In the wild there's almost no chance of it happenning. In fact, as a rule, I always wash my snakes when I bring em out to shows where kida handle them. Reduces your chances of salmonella transmission. BTW, I'm not saying that my snakes always sh**ty but it does happen occasionally and I don't see anything wrong with washing them.