Well, the day wasn't a total loss - I found the geocache I was looking for (Game Changing). I walked to the beach and went about a half mile upriver on the trail. There were a lot of twenty-something aged people. I didn't see any naked sunbathers, though there are many spots where you could probably find some privacy. Nothing is marked with signs. To get a group together, we would have to meet in the parking lot and go down together, or give out the coordinates and navigate there with GPSs.
The water was nice, I spent some time in the channel that runs parallel to the river. The rocks were slippery, and there is a lot of poison ivy in the park, especially close to the river. Alcohol is not permitted, but I saw a lot of people with beer bottles. It was very crowded being Sunday. When I left at about 3, the parking and all of the nearby side roads were full.
Maybe we should plan a Textile get-together to scope things out. To meet on a weekend, it might be best to wait until September, when it might not be as crowded.