When snails die don't they vacate the shells? If you think he's dead, and don't like hime anyways. Pluck him out and flush him. Who knows maybe in a few years you will hear about a mutant snail that someone flushed down the toilet and is now terrorizing the city.
You can pick up the snail with your hands, keeping it in the water, and gently push on the shell door. If it's firm and/or bubbles come out, it's alive. If all the guts start to come out, well, it's dead...and your finger will stink forever!!!