I think it's you guys who have been talking perpetual motion, is it?
If not, please explain part two to me, I don't see any difference:
What mechanism of energy conversion you use does not matter much, sails, a windmill that powers a propeller, etc., etc.: They all take slightly more energy out of the "wind energy", which is supplied by the current in the Boat B case, than they contribute. Even without friction/efficiency loss, they'd achieve a zero overall gain.
If wrong, where?
This is getting interesting.
You say: "... you may want to consider the energy is introduced by the current and wind."
no wind in TDW's riddle. Nor current in mine.
But you deny me use of my "apparent current", while you allow tdw's B use of his "apparent wind". No fair.