Berosus sp..
Size: approx. 5-6 mm. Photo: 08/07/12, 11 pm.

This very interesting beetle is Berosus sp. (26 species in the US). It belongs to the family of Water Scavenger Beetles. Adults can "breath" underwater by absorbing oxygen from the water through their body surface. Notice that the second and third pair of legs have swimming hairs on the tibia. 
bug guide (this photo):