I finally made a trip out to Widewater on Saturday morning after reading about it here. Despite the thousands of hikers, bikers, and other jokers there for God knows what, I found a parking spot right by the front of the parking area. I stopped to cast anywhere along the tow path that I could climb down to the rocks at the waterline. I was using a fast sinking poly leader on a floating line, so I don't think I was getting deep enough to catch anything until I got to the head by Lock 15. The water was so shallow and clear there that the sun had warmed it up and I could see and catch lots of bluegills. I only caught small ones, but I caught sight of some donkey-sized fish prowling around. I also went above Lock 15 and caught a bunch more there (they looked more like perch than bluegills to me, but hey, they eat a fly).
I'm planning on going back on a weekday with a full sinking line to try to fish the deeper part. If anyone has ideas on how to fish that better, especially in cooler weather, I'd love to hear them. I felt like I was getting short strikes in certain parts, but I couldn't convince myself they weren't rocks. The Potomac stretches around there looked good to fish too, but deciding where to start made my head hurt so I'll save that until I can go with someone more knowledgeable.
Basically, if you haven't been there before, don't go cause you'll hate it :)