Water boatman

Water boatman, as a type of insect, can mean: In the USA, Corixidae: these are mostly plant-eating insects but also eat small animals such as tadpoles. They inhabit ponds and slow moving streams, where they swim right-way-up near the bottom.
In Britain, it can also mean the Notonectidae (called backswimmers in the USA): these swim upside-down on the surface and are predators. They swim with two long legs that look like oars.