When I used to row crew in highschool, we used to do some races on the Potomac along the Georgetown waterfront - the current used to be vicious there, so I've always avoided that part of the potomac for paddling. I can't imagine deciding that it would be a good place to swim.
At Mason neck, I had my friend with a sit on top with me - I was more worried about her. Her boat's kinda slow & fat, so I didn't want to take her somewhere where we had to do a lot of paddling to see something pretty - which the Occoquan reservoir is... At Mason neck, to get up the creek we went to, you pretty much maybe paddle about a half mile up the Potomac which you can do next to the shore, and then turn up into the creek and the creek's literally like 2 feet deep in a lot of spots - so if it's high water & 4 feet deep that didn't seem too threatening. I posted the pics on facebook if you didn't see 'em - just generally a pretty day & calm water. Mason neck's also pretty much instantly gratifying - you don't have to go far and you're almost guaranteed to see bald eagles, herons, & a ton of other wild life.
I do enough seashore paddling in waves & rough water, open water crossings, and just jumping out of my boat & practicing self rescues & rolls that I know I'm at least not going to lose my head if something goes wrong (and whether or not not losing my head is enough to help me, probably depends on the circumstances). And I usually have just in case stuff with me... spare paddle (if not 2), dry bag, bilge pumps, emergency blankets, ropes, throw bags, lights, matches, water & food, knife on pfd for in case I get tangled in lines, etc.... All of which is normally ridiculous overkill on a normal paddling trip, but they just live in my kayak (my biking backpack is similarly ridiculous overkill). However, I avoid rivers like the Potomac or ones with whitewater when they're high.
I made sure to stick close to my friend, and had a throw rope handy and some rope in case I needed to tow her... But that sit on top of hers is probably wide enough that I didn't really have to worry about her flipping.
I might bike over to the Monocacy this evening to see if I think it's safe to paddle on one evening this week, otherwise I'll probably head out to triadelphia reservoir or brighton dam one evening.

