Sorry, been away ion holibobs... Yes. snails do come to the surface. It is thought that they do this to "migrate" to other parts of the lake. If you think about it, it would take them years to travel from one end to the other of most lakes
They fill a part of their shell with gas/air that makes them buoyant, float to the upper layers, and then get bobbed along in the tow. Once they get to where they want to go, they expel the gas and sink to the bottom again.
Another theory is that its to do with spawning.
If you have a garden pond, you may actually witness this. I can remember one morning seeing loads of them bobbing around and the goldfish trying to eat them. Come evening time they had all sunk again.