I travel the DC Beltway and I-95 between DC and Richmond constantly and I have never seen a towed boat hassled. That's not to say it won't happen though; I just have not seen it.
STAY OFF the Beltway and I-95 between 6AM and 9:30AM and 2PM and 6:30PM. The commuter traffic will not be friendly to you with that towed load, wide or not.
You could also come down 301 from Annapolis, but you need to check the 301 Bridge over the Potomac for width.