Thanks all, for weighing in. I've been talking about it more with my partner and a boat really does have some numerous advantages over a house. In particular, I'm attracted to having a weekend getaway I can go exploring in - something that isn't as expensive to buy or maintain as a house - and something not so far away that I couldn't just go down for the day. A lot of the vacation properties we've been looking at in the mountains are 3-4 hours away. There are plenty of marinas within a 2 hour drive.
A nine month vacation getaway suits me fine. In the winter I always want to go north anyway, to visit my family and go sledding and skiing. Winterizing a cabin is enough of a pain that I probably wouldn't visit my vacation home too much in the winter anyway.
As part of the point would be to get out of the city, I think we'd rather be in a quieter more desolate marina, which would hopefully be cheaper than a slip in Annapolis. I've only begun researching marinas - currently we daysail out of Belle Haven near Alexandria; several people have mentioned that we investigate marinas down near Solomons, which is only a 2 hour drive out of DC.