I don't know about snakes, but It works for me.
Seriously, In working with dogs we always start the pups in"small water" that they can wade across, then to "larger water" that they can swim in and easily see the other side like a small pond, and then to "big water" like a lake where they can't see the other side. I don't see any reason why the same "training" techniques would not work with snakes to get them used to swimming for a meal.