Also search the Internet for pictures of positive ventilation. Usually you figure out the prevailing winds (northwest in winter here) and put the "intake" in that direction. Then you put an outlet vent in the opposite end of the cover, and have some sort of fan going (a really spiffy install would have a means of sensing the humidity).
The reason for doing this is simple: a warm boat beneath a cold deck and a cold inner layer of air will condense like crazy and it will be miserable inside...a dripping, nasty mildewy mess you can't air out.
If you have adequate dock power and your boat can handle 50-60 amps, you can run those ceramic heaters, but liveaboard friends of mine ended up using the portable rad units that are filled with oil. They stay warmer longer, because the oil can get above boiling temperature...great for drying wet clothes!
Others get more elaborate and have a propane or diesel stove with a fireproof standoff for the chimney that goes right through the cover and frame. The heat going up draws the moist air right out the chimney, but of course you have to have an inlet or you can get sick or die in your sleep.
It's no picnic in the winter, but it CAN be done in relative comfort with forethought and planning. I recommend "The Warm, Dry Boat" for more ideas. It's not geared so much to winter liveaboards as it is to PNW/B.C./New England liveaboards, where the sea itself is unlikely to freeze. But the general principles are in there and it is those you must understand to have a positive, pneumonia-free experience.