Whirligig beetle

The whirligig beetles are a family (Gyrinidae) of water beetles that swim on the surface of fresh water.
They usually swim on the surface of the water if undisturbed, though they go underwater when threatened. They swim rapidly in circles when alarmed. They have divided eyes which may allow them to see above and below water at the same time. 
The family includes some 700 living species worldwide, in 15 genera, plus a few fossil species. Most species are very similar in appearance, though they vary in size from 3 mm to 18 mm in length. They tend to be flattened and rounded. Their shape is a good approximation to an ellipsoid, with legs and other appendages fitting closely into a streamlined surface.
Both larvae and adults are active predators. The adults carry an air bubble under their elytra, and can submerge for long periods if they want to. Mostly they move around on the surface in groups.