The key is to keep the boat warm all the time. Once it's cold its cold and take hours and hours to warm up again.
Last winter there was a few days/nights of -19C. Having the Oil Filled heater going full out and the 2 space heaters at 900watts I was able to keep the boat about 18C or so. But most nights I was on the boat with a tshirt on and the temp was about 21-22C.
I've learned from my first winter. I've done liveaboard for 3 summers and last yr was my first winter. Guess who is doing it again this winter