Male sirtalis, never. Female sirtalis, sometimes but not often.
http://www.thamnophis.com/thamphotos.../swimmingk.jpg
The female might even be using it to get away from the male when it gets too excited. The chase usually ends at the water bowl, with the female in it and the male outside it.
I think it might also have something to do with their swimming possibilities as babies, at least the male didn't have more than a small cup until about a month or so before I bought it.