We have an Eberspacher hydronic, which heats a glycol/water mix (just like antifreeze in your car, but a bit thicker, maybe 50/50 mix?)
This gets pumped around the boat, we have it going thru the hot water cylinder, and thru 3 heaters. 1 in each cabin, and a double size in the saloon.
The heaters just run a little fan like a PC fan.
If you get organised you can even bend up some stainless tube into a towel rail, and have a heated towel rail.
The heaters look like this