My GALS (Achatina fulica) accidentally discovered that they love to "shower" in the kitchen sink. So I set a little board across the sink, then I put a container with a lid right next to/halfway under the board. Then I ran the water lukewarm on a very low stream (very close to the edge of the container) and put the GALS along with some apples on the board. Then they crawled down onto the container, 1 or 2 at a time, taking turns to stick their face/head into the water, bending down over the edge to shower their back, then rearing up to shower their belly/foot. They'd keep doing it far past the point of being clean, so it wasn't for hygiene. I think it was for pleasure. Some went on another snails shell and showered at the same time, a few would "kiss" under the water... so they seem to really enjoy it.
It depends on what is in your tap water, as in, how chlorinated it is, etc. If you would drink lots of it right now without any worries, then it's likely that so can your snails.
Excess showering messes with their skin bacteria, so don't do it every day. Maybe once a week is safe. It's great to watch them enjoy it though.
I live in Hawaii and our tap water is the best water in the world. It's mountain water that filters through natural sheet rock for 20 years or something, it tastes so good and they never put anything in it like fluoride or chlorine or anything.
But ok thanks for the answer, I'll make sure not to do it too much. And yes it was adorable to see!