Alisoni – This can often happen to a child. A dream is very real to a child, and so it cannot see the difference between waking and dreaming.
It is difficult to know what associations you had with a snail, one that I had was that at the slightest touch it would withdraw.
But your dream seems to be one of exploration – one to see what it is like at the depths of a pool. And that means you were looking into the workings of your body, under the surface. And a snail could easily depict the subtle sensitivity within you.
Tony