Our last boat, in fact. I was stunned by the fact we could anchor(maybe we were on a mooring?) nearly in the shadow of the Washington Monument, jump on the subway and enjoy one of the best outdoor seafood markets around, right off the docks.
I hope Fiona and Iain are in the same place. They'll have a great time despite many of the spots they wanted to visit being closed. The Potomac is really very beautiful as you get upstream. You'll never forget Mt. Vernon from the water as you turn the bend.