This is a backswimmer, one of the true bugs (sucking mouthparts--see that beak at the front end?) in the genus Notonecta, most likely. Backswimmers swim upside down, on their backs (duh!)
The ones in our pond have pretty jade-green spots along the side and at the tip of the abdomen. I have no idea what they look like on the topside, since I only ever see them swimming upside down. The Mars-like surface behind this backswimmer is an old clay pot, broken, that's on the shelf of the pond about 8 inches under water.