Source : Webster's Revised Unabridged Dictionary (1913)

Water beetle \Wa"ter bee"tle\ (Zo["o]l.)
   Any one of numerous species of aquatic beetles belonging to
   {Dytiscus} and allied genera of the family {Dytiscid[ae]},
   and to various genera of the family {Hydrophilid[ae]}. These
   beetles swim with great agility, the fringed hind legs acting
   together like oars.

Source : WordNet®

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