water boatman

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various aquatic insects of the families Corixidae and Notonectidae, having long oarlike hind legs adapted for swimming.

Century Dictionary and Cyclopedia

1. n. The boat-fly or boat-insect, an aquatic bug of the family Notonectidæ: so called because these insects move in the water like a boat propelled by oars. They are more fully called back-swimming water-boatmen, and also back-swimmers, because they row themselves about on their backs with their long feathered oar-like legs. Some species are very common in ponds and brooks in the United States, and are often put in aquariums to exhibit their silvery colors and curious actions. N. undulata is a characteristic example.
2. n. An aquatic bug of the family Corisidæ. All the North American species belong to the genus Corisa, as C. undulata.

Wiktionary

1. n. Any of various aquatic insects of the family Corixidae that have paddle-shaped hind legs used for swimming.

GNU Webster's 1913

1. (Zoöl.) A boat bug.

WordNet 3.0

1. n. carnivorous aquatic bug having paddle-like hind legs