The Collaborative International Dictionary of English v.0.48:

Water beetle \Wa"ter bee"tle\ (Zool.)
   Any one of numerous species of aquatic beetles belonging to
   Dytiscus and allied genera of the family Dytiscidae, and
   to various genera of the family Hydrophilidae. These
   beetles swim with great agility, the fringed hind legs acting
   together like oars.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water beetle
    n 1: any of numerous aquatic beetles usually having a smooth
         oval body and flattened hind legs for swimming