We have little reusable chemical heat packs for snow skiing that tuck in a pocket or wherever, it would be easy to sew a pocket on the back of the glove to slide them in, they are handy in that you boil them to set them after use so you can reuse them and you have to snap them to start the reaction going again, you could sail until your cold then snap them and tuck them in the pocket, slide one down next to your nuts as well if you want. If you have a basically water tight glove and only need to stop water going into it, a stretchy band like used on dry suits pulled over it would seal it, I have an arm cover used for pregtesting cows that would seal it quite well so maybe you could check out a dairy supply company for gloves etc as they work in very wet and cold conditions.