Terrestrial snails move using a mechanism called adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit a force to the ground through a thin layer of mucus which is adhesive at low strains but otherwise flows like a liquid.
Obviously water snails move their foot in the same undulating way as their terrestrial cousins but adhesive locomotion can’t answer for their albeit small, velocity because there’s nothing to stick to. It seems the undulating motion of the foot deforms the surface of the water. And this generates a pressure that causes the mucus, which is sandwiched between the foot and water surface, to flow. In other words, water snails surf on waves of their own making.