sometimes the snake will sink and sit there. just because you want the snake to swim doesn't mean the snake wants to swim. We put a couple snakes into a CHEMICAL FREE pool last summer and they tended to just sit there until they felt like swimming, which was triggered by us taking a step back from the snake (they no longer felt safe in the open water). here's a link to see our baby feather swim http://snakes.lexxdog.com/feather/feather-swim.mpg
BTW if you do decide to take your snake swiming, make sure the pool is chemical free and stay near them so they don't hurt them selves and you can stop them from going into filters etc. I would not suggest taking them to a natural pond because of the bacteria in the pond and other unforseen issues.