Thanks to all here who helped with comments. I think I finally have a plan for my cabin heat. You can see my web site page here if you aren't sick of hearing about it already.
When I think of all the 6 and 7 figure decisions I've made in my career on pretty much a "Hey, let's do it this way." basis, I feel kind of silly at how much agonizing goes into making decisions about my own boat. If I'd charged for all the time I spent on this, it would probably have paid for the system three times over.
It's been a complicated thing to figure out though. Having my existing hot water heater at the other end of the boat and too low for natural convection to make the cabin heater water loop work without the current draw and sound of a constantly running pump took some head scratching. I think I finally have a good solution that will also give me a bit of heat in the head. You can see it at the link.