Weird.
We have big garden snails up the wazoo thanks to some dingbat Frenchman years ago, but they aren't aquatic as such. Aquatic pond snails seem to be a different breed, and don't live out in the air. I don't know what you've got.
The little black sea snails in the tidal ponds by the ocean have shells like iron. The land snails squish very unpleasantly if you accidentally step on one in the dark. The sea snails you can actually stand on if you're careful.