the only snails found on our shores that are suitable for tanks are cowries,some limpets....the best one is Nerita polita
not only they stay under water, doesn't knock off rocks like turban snails, pretty shells(probably no 2 shells are the same), easy to collect, its free and abundant.
https://www.google.com.sg/search?q=Nerita+polita&hl=en&client=firefox-a&hs=yqp&rls=org.mozilla:en-US:official&prmd=imvnso&tbm=isch&tbo=u&source=univ&sa=X&ei=j399UIHOHMPHrQfkooCgAw&ved=0CCAQsAQ&biw=1024&bih=414