I’m puzzled too. Yes, the water is supporting the boat, but what on earth does ‘you must plus the force of the air’ mean? If they are talking about air pressure, that isn’t part of the equation because the air pressure is constant inside and outside the boat. It isn’t as though the boat surrounded by a vacuum but was full of air and it was exerting a downward pressure. You could also add the force of gravity if you were looking for external pressures. But the bottom line is, the boat floats because of the displacement of water. That is why a metal boat will float – because the shape of the boat displaces water equal to or greater than the weight of the boat.