I'm interested in living aboard year-round, and to keep my job I'll have to do it in/near Boston, MA. I'll probably buy a sailboat between 35 and 40'. Winters here are cold and snowy, which leaves me with a few questions.
There is a Yahoo mailing list group for Boston Liveaboards. You might want to join. They have a lot of social activities, mostly centered around Constitution Marina that I'm sure you would be welcome to attend.
I live aboard in Annapolis which has a similar climate, just as cold only not for quite as long.
1) I'm fairly certain I should invest in central heat
Definitely. Reverse-cycle heat pumps (A/C and heat) can make heat reasonably well until the water temperature (not air temperature) gets down to 40F or a bit below. They freeze up and stop working at all at some point. Still, for a liveaboard I chose to put heat pumps in my boat.
For temperatures that stay cold long enough to make heat pumps ineffective most liveaboards depend on electric heat. It works, the heaters are reasonably inexpensive, and there are no fuels to fuss with. Cons: won't work if the power goes out and take up room.
There are a number of other approaches including kerosene and propane heaters. The folk at Constitution Marina will surely fill you in.
The most home-like central heat is forced air diesel-fired heat. The initial equipment costs around 2 - 3k from manufacturers like Espar and Webasto. They burn about 3/4 gallon / day to keep the boat pretty toasty. This was my choice (in addition to heat pumps) to keep the boat warm in the depth of winter.
2) I hear over and over that humidity is a huge problem.
Buy or borrow a copy of "The Warm Dry Boat" by Roger McAfee. Humidity can be a challenge. The three most important things you can do, in order, are to insulate the boat, ventilate, and keep the boat warm. A little preparation will save you a lot of discontent.
For lockers, some ventilation between the locker and boat and insulation against the hull will help a lot.
3) How do you keep the tanks from freezing? Are there products for this that keep them warm? I know there is heat tape for the outside water line (and at least one marina near here requires it, which leads me to believe they provide water year round). If it's like heat tape, what's the power requirement?
If the boat is warm the tanks will be fine.
If your marina has winter water heat tape is their problem. You really don't want a continuous pressure water connection at any time -- if a hose ruptures inboard you have a constant flow of water into your boat. We call that "sinking." *grin* Generally marinas with liveaboards will have hose parties once a week during winter to string hoses together to some inside hose bib to fill tanks. The alternative is lugging a 5 gallon jug of water every time you go to the boat.
4) I'd like to use the onboard shower year-round if possible, but my understanding is that most liveaboards use the marina showers in the winter... is it practical to use it in the winter, and what might I need to do to it?
I shower onboard and haven't found it to be a problem. I clean the head once a week (10 minutes). The head on my boat is at the base of the companionway across from the galley. When I shower or cook I slide the companionway open a bit for additional ventilation. No problem.