Shiny black, brown to olive beetles, sometimes with yellowish marks. Body is streamlined, oval, with the narrower end at the head. Antennae are threadlike. These beetles hang head downward, with the tip of the abdomen protruding from the water surface. The hindlegs are fringed with hairs and flattened for swimming. When swimming, they kick both hind legs simultaneously (not alternately). The swimming method helps distinguish them from the similar water scavenger beetles, which are in a different family.
Like other beetles, they have membranous hindwings that are covered by forewings that are thick, heavy shields (elytra). When the wings are closed, the elytra create a line straight down the back.
Larvae, called “water tigers,” are elongated, flattened and can be 2 inches long. They commonly come to the surface to draw air into spiracles (like snorkels) located at the hind end of the body. There are 3 pairs of legs, and the jaws are strong pincers that are used to grasp prey.