The Collaborative International Dictionary of English v.0.48:

Sea snake \Sea" snake`\ (Zool.)
   Any one of many species of venomous aquatic snakes of the
   family Hydrophidae, having a flattened tail and living
   entirely in the sea, especially in the warmer parts of the
   Indian and Pacific Oceans. They feed upon fishes, and are
   mostly of moderate size, but some species become eight or ten
   feet long and four inches broad.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea snake
    n 1: any of numerous venomous aquatic viviparous snakes having a
         fin-like tail; of warm littoral seas; feed on fish which
         they immobilize with quick-acting venom