This snail, apparently feeding on the abundant algae (cyanobacteria) growing on the rock, is a Physa species. They are very abundant along the shallow edges of a large creek that runs thru the park near where I live. Despite the fact that they are aquatic snails, they actually breathe air via the inner roof of the mantle cavity that functions as a lung. And because of that, they need to surface frequently to take in fresh air.
Often, I find them at the boundary between water and air. They leave the water frequently and crawl on the wet surfaces of the rocks. The snail near the top of the picture below was completely outside the water, while the lower one was partially out.
Their amphibious lifestyle seems to give them the best of both worlds: their lungs enable them to leave the water to graze on the algae-covered rocks, while the proximity of their aquatic home assures that they are unlikely to run out of water.