Moved from Hydrochara. I have Ciegler (1), and have just looked at it in detail--I did not have it with me when I posted the photo--this still seems to key to Hydrochara soror, though the key is tough (palpomere 3 1.5X length of palpomere 4, versus 1.3X!). Based on the ranges she gives, too, H. soror seems to be plausible, H. obtusata being found northward of Maryland, and H. spangleri (close in the key) found in Sandhills of South Carolina, not in my area.

Ciegler does say for H. soror: "ventral surface of many specimens rufopiceous", meaning that not all are so-colored, I guess.

You're correct in that this has the same to characteristics of keel and palpi as H. soror. But because of the significantly darker ventral color I'm hesitant to say it actually is H. soror. From Downie and Arnett it could also key to H. obtusata, the species Tom has posted. And H. obtusata has a dark ventral color like yours. But everything I've found on H. obtusata seems to point to a more northly distribution (nothing south of Maryland). I don't have Ciegler's, Water Beetles of South Carolina, which might provide some clues (but I'm hoping to receive Phillip's volume on SC Scarabs for Christmas :) So unfortunately, I'm of little help here. But with only 8 or so species in NAmer and your excellent ventral photo, I'm sure a Water Beetle expert (like Dr. Andrew Short) will nail this one in a instant.