Not to derail this highly scientific thread but....
...I have these 12v car seat heaters and they do their job. Warmer than body heat. Its a ziggy plug with off/warm/hot switch and a wire going through the cushion....
...The rest is history.
I guess one could use a solar shower, too. But maybe...
Surfboard leash plugs would be a starting point. Classic longboards use the ones cyclone mentioned. You can also make a glasson leash loop from ud rowing, it could be very tiny so not so much added drag.