All they had to do..
..after getting into the water, was to have the three guys hold each other close and form a makeshift platform with their shoulders, the guys would then inhale as much as they can and hold their breath, while the lightest girl gets on top of their shoulders and gets on the boat, and then lowers the ladder.
Simple as that.
It was so annoying going through this movie and they don't even attempt a shoulder stand, and that is literally the first thing you would try in this situation.