Source : Webster's Revised Unabridged Dictionary (1913)

Boat bug \Boat" bug`\ (Zo["o]l.)
   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}.

Source : WordNet®

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