well, again, the boat displaces more water when more people get in, because it sinks further. Anytime mass changes and volume does not, then you have changed density. That is what I'm talking about. The density of the boat/cargo/people system is dependent upon all of the factors adding mass--not just the boat itself. If you get too much weight in the boat, then it will eventually sink all the way. The boat's bottom is shaped to make sure it displaces more water the deeper it goes--which is what i said previously (shape matters). The farther the boat bottom sinks, the more volume is exposed to the water surface for bouyancy. Again, the displacement potential is due entirely to the shape/volume of the object. The farther the object sinks, the more water is displaced. Turn that same boat on it's side and load it with the same amount of weight, and it will sink to the bottom--because there is not enough surface exposed to displace enough water.

BTW, a 1 cubic foot chunk of wood will displace more water than the foam because it sinks further into the water--again, because of mass, which is due to density, not just volume. If you push the foam down into the water as far as the wood sinks, then they will both displace the exact same amount of water. Again, the wood sinks further because it weighs more--which is what you are saying. What I'm saying is that it weighs more because of it's density--which is one controlling factor in bouyancy. Archimedes described density because he was given a job by the king to make sure his crown was solid gold. He said that different materials would displace different amounts of water, if they all had the exact same mass.

However, the density factor can be overcame by adding more volume to the interface between the object and the liquid it is floating in.