On your under floor heating.....
I installed it in our house extension. It's a water overlay system heated through a domestic gas boiler.
It sits on hardcore, 4 inch insulation, min 4 inch slab and an inch of screed.
Though there is an alternative 'joist' option, (which wasn't there couple f years ago when we did ours)
Many systems are installed under the slab, they do take a long time to warm through properly (couple of says) but act as a heat store. You don't have that choice.
We were able to tile directly over the pipes that sit within boards with pre-made grooves. You have to consider the floor finish - tiles are about the best conductors of heat.
The room (10m x 4m) heats up within an hour controlled in 3 zones by stats.
The system runs much less hot that conventional radiators (appro. 27 degrees to 60/70).
Where heat is not required, under kitchen units for example, they provide 'blank' boards to maintain equal height throughout.
I can't thing of a reason why it couldn't be run from an on-demand heating boiler on a boat - diesel perhaps?
Here's the system I used
The company provided everything for a self-install which I did myself. So, cost wise it was very reasonable. There's is a good set of instructions, detailed online instructional videos and prompt telephone assistance for when I got in a muddle.
If you call them I'm sure they would offer some helpful advice.
BUT, you will have to consider (at least):
access will be required to the bilge at some point.
A boat's movement / stability when in rough water or hitting a lock gate etc. (You don't want pipes coming asunder.)
Electric UHF is simple to install bit is (or certainly was) mighty expensive to run.