The Collaborative International Dictionary of English v.0.48:

Water boatman \Wa"ter boat`man\ (Zool.)
   A boat bug.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Boat bug \Boat" bug`\ (Zool.)
   An aquatic hemipterous insect of the genus Notonecta; -- so
   called from swimming on its back, which gives it the
   appearance of a little boat. Called also boat fly, boat
   insect, boatman, and water boatman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water boatman
    n 1: carnivorous aquatic bug having paddle-like hind legs [syn:
         water boatman, boat bug]