So I was talking about hydro generators that go in the water rather than a wind turbine that rely on direct wind. I prefer the former as it doesn't muck up the aero on the boat (sails can rob it of power with their dirty air) nor does it get in the way of the deck space that is often limited on a smaller boat. You could use both but I am not fan of wind turbines except at anchor.

With a normal sail boat its common to have more wind that you can safely handle with the boat, you can use the drag of a hydro generator, which goes in the water only when you need it, to help slow the boat down rather than reefing or reefing as much (intentionally making the sails smaller). As wind is the only thing generating motion for the sail boat, its free energy that you otherwise would not be using.

Motor boats rely on the motor to move them, so any drag means the motor has to work harder, imagine dragging a drogue (water parachute used as an anchor), its the same thing, its no longer free energy and you are actually spending more energy moving the same distance as the hydro generator doesn't generate as much energy as it costs to use. Its the same problem for a wind turbine, as that increases aero rather than water drag for the motor boat, requiring more effort from the motor for the same speed & distance.

Motor yachts would be better switching their engine over energy generation as a generator for recharging the EV batteries. Indeed there are some bigger motor yachts that do exactly this as a backup to their solar and dock derived power.