change this comment also, just so it does not lead to search confusions....

This is a beetle, Hydrophilus, of the beetle family Hydrophilidae. It is NOT a bug, which the two "answers" you provided refer to. Water boatmen are members of the bug family Corixidae; backswimmers are members of the bug family Notonectidae. This is probably Hydrophilus triangularis.

With all due respect, I amended my first answer because I knew it to be incorrect and it is therefore null and void of any relevance. Why you feel the need to chastise me for a previous amendment is beyond me. As to my second answer, pardon me for daring to throw out an educated suggestion. I should have been more questioning perhaps.

I was only answering objectively; if you perceive chastisement, it's your problem.

It is not my fault if that is the way you made me feel.

I should have put a ruler next to it. It is quite large. I still have the specimen so will measure it to be precise but it's not like the small water beetles that are perhaps 1 to 1.5 cm in length. This is probably 5-6, close to the 2 inches that I used in the description. That should preclude similar smaller species.

All the species of Hydrophilus are about the same size, and they're all large! What one needs to see are the foretarsi, the venter and the male genitalia to be sure. H. triangularis is the most common species in the E US; the only other species you would have there would be H. ovatus, but I can't see the prosternum to be sure.

Did you see the accompanying ventral photo? Will try and get some more pix.