WAYS:Boats p.1
Boats:

As long as humans have observed objects floating on the surface of water they have attached their bodies to them to transport themselves as well as goods across stretches of water. The technologies and materials used in boats are innumerable. The dugout canoe is an obvious example of a simple but effective design. In Vietnam, large round baskets are popular. Whether a basket weighing not more than a man, or the Seawise Giant (later known as the Happy Giant, Jahre Viking, and Knock Nevis)which fully laden weighed up to 564,763 metric tons, all water born vessels composed of materials heavier than water rely on one very simple principle: displacement. 

Archimedes of Syracuse (Greek: Ἀρχιμήδης; c. 287 BC – c. 212 BC) explained this principle in On Floating Bodies:

Proposition 7. A solid heavier than a fluid will, if placed in it, descend to the bottom of the fluid, and the solid will, when weighed in the fluid, be lighter than its true weight by the weight of the fluid displaced.

Displacement means things that take up a volume but weigh less than the amount of water or other liquid that would take their place will float because they are relatively lighter than the fluid. So, shape counts more than weight when trying to get things to float and that is the key to designing hulls.