I'm a part-time liveaboard, using my 38' Beneteau as a mid-week hotel room, currently in Baltimore harbor. I use a couple of electric space heaters, turn one off when not on the boat, never had a problem with freezing even on sub-20 degree nights. I have these Vornado heaters that have variable speed fans, they run slower and faster depending of whether they are heating or not but never go off completely. That gives some constant air circulation. Also the heating elements are lower temp, they don't give that hot blast so you don't get that hot one minute, cold the next, feeling.
I have two freshwater tanks, one under the forward v-berth one under the aft cabin. I don't try to heat the aft cabin and the water hoses for the swim shower run through the transom area so to prevent freezing, I drain the aft tank, dump in a couple gallons of cheap vodka and "purge" the hoses from that tank out to the swim shower. In the spring I flush out the tank and lines with fresh water. The previous owner had winterized both tanks with potable anti-freeze and it took weeks to get rid of the smell. One flush does it with the vodka and there's no smell. This is my second winter aboard and I'm very comfortable.