The Collaborative International Dictionary of English v.0.48:

Hydrophid \Hy"dro*phid\, n. [Hydro-, 1 + Gr. ? a small serpent.]
   (Zool.)
   Any sea snake of the genus Hydrophys and allied genera.
   These snakes are venomous, live upon fishes, and have a
   flattened tail for swimming.
   [1913 Webster]