Many thanks to Thomas for his report on the Watt&Sea. I had much higher expectations, based on all the adverts and testimonials one often reads about.
With regard to Ian's comments that the Santorin, Maramu, and early SM had the hydrogenerator, it should be mentioned that as late as 2002, it could be ordered on the Super Maramu 2000 during production.
So Peregrinus has one, for example. At 4 knots, it produces nothing. From about 5.5 knots, it will supply the boat's consumers (autopilot, etc.), so it will maintain charge, but if you want to see a 10A charge going to the batteries, you need to be doing 7 knots or more. We don't really rely on it, but on an ocean crossing it could save your bacon if other power sources become disabled somehow.
at anchor, Siracusa (Italy)