Two of these beetles were standing on these yellow eggs under a rock in the creek. In this photo I've flipped the rock over and placed it in a bowl with water (so eggs and beetle are still in water).

Cheryl Barr tells me that the females are under water during most of their short adult lives, whereas males can be found above the water. So probably this one is a female. The eggs look right for this species as well. Interesting that the beetles from under the water have the grayish stuff on their pronotums and undersides -- something to do with staying under water? I found a number of these above water too, none with the gray coloring. I didn't try to rub the gray stuff off, but it looked as if it could be rubbed off.