The Collaborative International Dictionary of English v.0.48:

backswimmer \back"swim`mer\ n.
   any of numerous predaceous aquatic insects of the family
   Notonectidae (such as Notonecta undulata) that swim on
   their backs and may inflict painful bites; -- also called
   boat bug. --[RHUD]
   [WordNet 1.5 +PJC]

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):

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