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 use an electric vest on the bike and on the boat. It's is just like getting inside a toaster. I have had it soaked in rain water and it worked fine. It is 12 volt and you need a thermostat controller or you will be turning it on and off.
The vest it self is made with thinsulate and is warm by itself.
I wear pollypropolene long underwear next to my skin, vest, light wool sweater and my leathers.
On the boat if I get a chill I just put it on and in just a jiffy I am warm. I am not sure what salt water will do.