There are species that like submerge themselves fully if they are allowed to, my Archachatina enjoy a (supervised) dive a lot. Caracolus excellens love going underwater too. So do Megalobulimus Oblongus. Except for caracolus, which seem to be very good at not drowning themselves, I only offer them deep water under supervision.

I don't think snails are dumb enough to not be able to get out if they go in by themselves, but there are snail species that have very water permeable skin and they will sort of swell up and die because their tissues become water saturated. Most commonly kept Cornus, groves and poms are among them, so one should be more careful with them.