Dorsal view to match the ventral view of this individual previously posted.

(Thanks so much to Andrew Short, who noticed that I had confounded images of two different aquatic beetles during the editing process. I've now sorted them out by going back to the original sequences and paying careful attention to the anatomy.)

Found stunned under a light. Placed in a tray for photography, where it swam rather languidly. Later released in an aquatic habitat.

With great trepidation, I have attempted identification of a couple of aquatic beetles. My reasoning follows. This beetle has a prominently keeled sternum. Aha! I think that means it is a Water Scavenger, Hydrophilidae. (Many, but not all genera have a keel on sternum, I think.) Given the large size, about 18-19 mm, I poked through the species descriptions in Ciegler, Water Beetles of South Carolina (1) and found that only two genera in the family are in this size range: Hydrochara, 12-19 mm, and Hyrdobiomorpha, 13-17 mm. The latter has a long spine extending posterior to the keel, the former does not. The photo of the ventral surface shows a short spine. This should be, then, Hydrochara. (Other similar genera are: Tropisternus, circa 10 mm, and Hydrophilus, 32-40 mm.)