Ventral surface, showing sternal keel and spine projecting back over abdomen. This, and the large size, are consistent with Hydrophilus. Note, also, that the middle leg is long and powerful, as well as the hind leg. I believe this is a characteristic of this genus.

Found at a light, rather stunned, but still kicking. Photographed, at the site I found it, and also, kept chilled for some studio shots later.

With great trepidation, I have attempted identification of a couple of aquatic beetles. My reasoning for this specimen follows. This beetle has a prominently keeled/spined sternum. Aha! I think that means it is a Water Scavenger, Hydrophilidae. (Many, but not all genera have a keel on sternum, I think.) Some predaceous diving beetles (Dystichidae) are similar in size, but do not have the keeled sternum. Given the large size, about 30 mm, I poked through the species descriptions in Ciegler, Water Beetles of South Carolina (1) and found that only one genus in this family is in this size range: Hydrophilus, 32-40 mm. Plus, that genus has a spined keel, seen in the photograph of the ventral surface. This is perhaps H. ovulatus, based on the rather oblong shape, as opposed to the more linear shape of H. triangularis. (1)

Comments on another image indicate this is Hydrophilus ovatus.