If you watch the smaller snail species, you may catch them actually gliding on the underside of the water tension ! I've got some vid of it somewhere, if I can ever find it. I've seen ramshorns do this, bladder snails do it, and I have seen mystery snails do something very similar though they are too heavy to actually suspend themselves from the water tension alone. Usually they have a grip on a bit of plant or the glass, but most of the foot will be on the water tension and the mouth will often be grabbing at some floating bit of something.
First time I saw a snail crawling on the underside of the water tension I was sure I was seeing things but I've observed it many times since. Not quite swimming, but not quite the usual method of getting around either. My mystery snails sometimes float around for a period of time and I've seen small ramshorns do that too. I get a big kick from seeing the big mysteries let do when they finish breathing to sail to the bottom of the tank. Like rocks falling but with a bit more control.