I mean sure, maybe there is no difference. But then, like the plane you’re better off not using the sail and just pointing the fan the opposite way so it pushes the boat fwd.

The engine on a plane pushes the airplane forward. The reversers redirect a portion of the thrust forward slowing the aircraft down. The best thrust reversers get MAYBE 50% efficiency. On the boat there is no reason to throw out half the fans thrust.

That’s combined with the fact that the sail isn’t an effective thrust reverser and that the worst case efficiency is negative. If you don’t redirect enough air backwards you won’t even overcome the fans thrust let alone produce a net reverse thrust.