Yes, snails need a bath occasionally. It's very easy to do. It will not harm them and is actually good for them. Most snails seem to enjoy a bath once every week or two. Plus, with their shells clean they look nicer too!
what you'll need:
+ Bowl full of substrate
+ Misting spray bottle
+ Room temperature water - not hot/not icy
+ And the snail of course!
Take a bowl and full it with dry substrate. This absorbs the extra water from their bath and cushions them if they should fall. You'll also need a misting spray bottle filled with room temperature water, set to mist instead of a straight spray. Avoid fliping your hand over when handling your snail. Your hand should stay slightly curved and palm up. Be careful not to spray water into their lung hole or you may drown them. You won't need much water. If the snail is crawling away and seems stressed, just put it in its house and leave the bath for another day.
Mist your hand then pick up the snail to be bathed in your left hand, or right if your left handed. Make sure its stuck on your hand. Holding it very low over the bowl of substrate, mist it's shell with the spray bottle till it's damp then using only your finger, very gently in a circular motion clean all of the snails shell. Depending on how dirty the snail is you may need to spray and gently rub several times. Finally, spray the snails shell a few times to make sure you have the dirt off then put the clean snail back in its house to do whatever it pleases.
Never bath your snail in anything but room temperature water - not hot but not icy either.
Never use anything but your finger to bath your snail. Anything else is to rough and may damage their shell.
Also, never bath baby snails, their shell is to fragile.
Snails with a broken shell - depending on where it's broken and how bad - should not be bathed either.