Since a sail is a kind of airfoil, why couldn’t a sail be devised for a boat with sufficient lightness such that lift could be added to velocity by trimming the sail to fly the boat as a plane? In the air one should be able to reverse the trim to descend back to the water. With such a sail, one might need to deploy an anchoring parachute like the tail of a kite for stability, and we’d need to work out how to steer, for we know a boat can’t be controlled without the resistance of water.