We have a Nanni too, Mark.
I wouldn't dream of heating the water by running the engine at whatever revs, because you're not putting a load on it and the consequent glazing of the bores will eventually cause you problems.
Yes, I know the hire boats do this but they also spend a lot of hours at high revs which probably offsets this.
I found that our calorifier had connections for 230V on it and I wired this in to the 230V system.