>>212406172 (OP)In theory the amount of upthrust caused by the air in the boat would be equal to the boat's buoyancy when used normally. That is, if the boat would float with those two men on top of it, then the upthrust caused by them carrying it underwater should be enough to overcome their weight. However there's nothing proving they didn't let a little bit of water inside, we don't know that the entire volume of the boat contains air. So it's plausible at least, but would be difficult to do in reality.