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.)