I've been thinking about heated clothing too, like the kind bike riders use when they ride motorcycles in very cold weather. It's low voltage so you don't have to worry about electrocuting yourself I don't think
But I would make sure before I tried it on a boat.
I'm concerned about hypothermia too of course. Right now when I get really cold I go in the house and take a warm shower and that warms me right up, but that probably wouldn't be possible on a boat. The next best thing might be warm drinks and sitting near the heat, or some kind of an electric blanket, but I don't know how much electricity that would use.
I've heard (read ?) that there are emergency suits that some people carry in extreme environments to respond to hypothermia, I assume they are electric. I'm not talking about the survival suits for life raft deployment, I'm talking about an emergency suit you use to bring people out of hypothermia. I remember at least one boater who claimed to have one on their boat.
If it's really cold I sometimes use the little chemical packets that heat up when you open them up. They are not a long term solution, but it's amazing how good warming your hands up can feel when you are really cold, especially if you need to work on something with your hands.
I almost forgot one of my favorite things - a warm water bottle. Yes, straight out of the 1930's, a plain old rubber water bottle that you put warm water in and then keep it with you. Best used for camping when you are going to sleep, but might be useful in the cockpit of a boat too.