The Collaborative International Dictionary of English v.0.48:

Water bug \Wa"ter bug`\ (Zool.)
   (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
       Belostomatidae. 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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

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, water bug, Blattella
       germanica]