Hydro-electric watermill power system..??

IT IS A WASTE OF TIME AND MONEY.

Simple answer.

 

If you really want to do something foolish that isn't already done well, like just having a prop regen or towing a prop driven generator, is to tap the energy from the waves.
So instead of having the boat pitch up and down, you can capture the energy and have it pitch less.

 

Yeah, this is what I figured would be the case. If it was worth doing someone would have done it by now.
Apparently the towed propeller types, like Watt & Sea, don't make a whole heap of power, unless the yachts are racing hard.

I agree that solar PV is probably the best way, but on smaller boats there is not much real estate to locate them, as there is on larger boats.

Whihc is why most boats have them mounted virtually outboard, aft of the stern, on a frame like davits that overhangs the stern and is raised above head height.

The problem with the coachroof is that half of it is usually shaded, and any panle in shade linked to other panels drops the total charge generated.

I'm uncertain of the electrical/physical phenomenon that causes this, but I know it happens, which is why you need a MPPT with multiple 'strings' so each panel can be on its own 'string' and thus not affect the charging capability of the other panels if it's in shade.