Not sure if this will help you or not but another reason they sometimes hang out in the water bowl is to relieve tension after a big meal. Did your snake recently eat?

Also keep in mind that they sometimes just feel like it. If your husbandry is good and the behavior doesn't persist for man than 2-3 days then I wouldn't worry about it. However if the snakes remains in the water bowl day after day I might treat the enclosure with PAM just in case.