Source : Webster's Revised Unabridged Dictionary (1913)

Water bug \Wa"ter bug`\ (Zo["o]l.)
   (a) The Croton bug.
   (b) Any one of numerous species of large, rapacious, aquatic,
       hemipterous insects belonging to {Belostoma}, {Benacus},
       {Zaitha}, and other genera of the family
       {Belostomatid[ae]}. Their hind legs are long and fringed,
       and act like oars. Some of these insects are of great
       size, being among the largest existing Hemiptera. Many of
       them come out of the water and fly about at night.

Source : WordNet®

water bug
     n 1: a true bug: large aquatic bug adapted to living in or on the
          surface of water
     2: small light-brown cockroach brought to United States from
        Europe; a common household pest [syn: {German cockroach},
        {Croton bug}, {crotonbug}, {Blattella germanica}]