Yes, the upper Potomac is a floating blind system. Pick you spot and do not hunt near others and try and stay away from hiking areas and houses on the VA, WVA side. You can even beach the kayak or boat as long as it is still on the wet river bottom.
Certain areas like Brunswick and parts of Montgomery county are off limits. Brunswick owns the bottom of the river and Montgomery has a no shooting ordinance on part of the river down there. You getting very close in Pennyfield where to no shooting zone is. Call over Echo lake and ask a ranger. If they give it to you let me know, I will call if they do not help. Let me know what they tell you.
You can anchor anywhere else on the bottom of the river. Some island are public, they small ones and a few big ones. Most big ones that make the map are private. There is no available list, just hunt smart. Throwing your anchor on The Canal or private land can get you fined, if they come check you. Many wardens would warn you unless you act like you are breaking other game laws.
"If you can't have fun doing it, it ain't worth doing, or you're just doing it wrong."
River hunting in my Wilderness Systems Kayaks; Commander 120 and Ride 135
My Blog archeryrob.wordpress.com Confessions of a fisherman, hunter and tinkerer