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: Webster's Revised Unabridged Dictionary (1913)
Boatman \Boat"man\, n.; pl. Boatmen.1. A man who manages a boat; a rower of a boat.
As late the boatman hies him home. --Percival.
2. (Zo["o]l.) A boat bug. See Boat bug.