Yes I don't really see how we can be discussing buoyancy and boats without discussing Archimedes principle.
In physics, buoyancy (pronounced /ˈbɔɪ.ənsi/) is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter.
Type "Archimedes principle" into Google and the first entry that comes up is the Wiki article on buoyancy.
As RAZD says displacement is the key criteria here. If the weight of the water displaced is not equal to the weight of the boat and it's contents the boat will sink. Thus the shape of the boat, to maximise displacement as weight increases, is absolutely crucial. A simple raft is a desperately poor design with this in mind.
That is why huge ships are the tapered shape that they are. Obviously.
The legends goes further and tells that Archimedes was so excited with his discovery that he hopped out of the bath, and rushed naked into the street yelling triumphantly, "Eureka!" "Eureka!"