We have aquatic snails in our fish tank, rather by accident. They came in on one of the plants we bought for the aquarium, and they multiply like crazy.
As someone who specialized in marine invertebrates for my biology degree, I rather like these little molluscs. They keep the sides of the tank spotlessly clean. They move surprisingly quickly (for snails). There are, however, often too many of them—they outnumber the fish by at least an order of magnitude—so we occasionally have to scoop some out and, uh, send them to greener pastures. (I have no doubt that things are greener, not to mention sludgier, in the Greater Vancouver sewer system.)
The snails perform one stunt that I hadn't seen before. Often they will make their way to the top of the aquarium and then "surf" upside down on the underside of the water's surface, riding the currents created by the tank filter. Sometimes one will circulate around for several minutes, its muscular foot flat against the surface tension, then it will either stick to a side or drift down to the gravel at the bottom to start trucking its way around again.
It's pretty cool, and probably the fastest these creatures will ever travel in their lives. Unless they get to visit the sewer someday, I guess.