Ocean snails breathe using gills while freshwater snails breathe using lungs. Ocean snails can have one or two gills, and the gills allow the snails to remain completely submerged all the time. Freshwater snails have something that resembles a lung but is actually a pulmonary cavity, and these snails may surface for air or, depending on the type of snail, remain submerged all of the time.
Some of the smaller freshwater snails actually suck in water and extract oxygen from that water. Others will create a water bubble that contains oxygen. This bubble not only provides oxygen but helps the snail to float in the water.