A boat floats according to the principle of displacement. The boat sitting on the water displaces a certain amount of water. That means that it pushes down on the water, and the water that would normally be where the boat is gets moved aside temporarily. The mass or weight of water that is displaced is precisely the weight of boat that will be supported or "floated". This is why metal boats float, when intuitively you would think they would sink. As long as no water can get inside to ruin the displacement trick, they float.