Most of what you list looks to me to be workable, though I think I fell asleep about 1/2 way through! I am thinking about much the same as you.
Remember that a boat deprecates. If you get an old boat that is mostly or all the way deprecated at least it will stay about the same. I was lucky and sold my house before the housing bubble burst, I made a big chunk of change, but if you subtracted the property and school tax I really did not make much. Just remember that a boat will have double the maintenance of a house, and drop in value. As long as you are OK with that then you should be fine. Financing is tough in the boat market as well now.
Biggest issue is what does you spouse think? That is the biggest complicating factor. If she/he is not 100% on board you might want to rethink the whole thing. In my case I am divorced so it is not an issue. I think I finally have my children excited about the whole concept.
From reading here it looks as though DC has some good live-aboard options. The best marinas will be about an hour commute out of DC proper, but that will likely be true for an apartment that would be in a child friendly neighborhood anyway.
Just don't think it will be cheaper than a home or apartment. About the same I think. Tool thing I understand fully, what am I going to do with a big air compressor on a boat?, where will the table saw go? Waht do I do with my rolling tool chest? But I am planning on keeping my garage for at least a couple of years, in fact the only reason I am where I am now is that I can rent a garage.