Interesting, but I don't think you really need it. If its warm enough to sail it is warm enough to start most Yanmars. Remember, your engine is inside the boat. If you have a reverse cycle AC, warm the whole boat.
Now if you do decide to try this, go to your local John Deere tractor dealer and see what they use for a warmer. The 650-1050 series (16-30 Hp) from the 80's were made by Yanmar and similar models may be made by Yanmar now.