There is a wind and water charger device that has both an air
propellor and a water propellor. Mounts on the transom and either
swung into the air or into the water with the appropriate prop fitted.
I've got Leece Neville alternators on the engine - 24v 95A and 110A -
and changed the pulley size to improve charging at lower revs, but not
exceeeding the max rpm at max engine speed.