The Collaborative International Dictionary of English v.0.48:

Sea snail \Sea" snail`\ (Zool.)
   (a) A small fish of the genus Liparis, having a ventral
       sucker. It lives among stones and seaweeds.
   (b) Any small creeping marine gastropod, as the species of
       Littorina, Natica, etc.
       [1913 Webster]

WordNet (r) 3.0 (2006):

sea snail
    n 1: small tadpole-shaped cold-water fishes with pelvic fins
         forming a sucker; related to lumpfish [syn: snailfish,
         seasnail, sea snail, Liparis liparis]