Look at that Escargot!
Okay, yes, it's an old joke! But some people don't think snails in their pond are too funny. Snails that originate in an aquatic environment do not survive in the desert outside the pond. The aquatic snails feed on algae and dead plant material (pond detritus), and are therefore considered a beneficial critter in that environment. There are very few varieties that feed on living aquatic plant material, and we only rarely see these varieties. If you do see them, they will most likely be eating your water lilies.
What should I do about snails in my Phoenix pond?
Don't worry about the pond snails. They are pretty much relegated to life inside the pond. AND it's great fun for young children to hunt for them in the rocks and plants -- it's an activity that can keep them busy for hours!
If you have snails in your garden, on the other hand, the Master Gardeners can help you out here, because those are a nuisance.