Note the tidepool in front of the log; very few snails remain there, and they're scrambling for the edge. Behind the log, on higher ground, the snails are so thick that in places they hide the stones.
They do the same thing in my aquarium; they spend at least half their time clinging to the walls above the water level, and sometimes leave the tank and go for walkies on the counter. Sometimes I find one behind the tank, dry as dust, completely sealed in, looking dead. I drop him back in the water, and he wakes up - and heads for the top again.
They don't eat out of the water; just sleep.