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]