There are a few things that determine whether or not a boat will float, such as its shape, its load, and where the boat is sailing. First, let's talk about density. Density is a measure of how much mass of something is packed into a certain volume. For example, iron is denser than wood because a volume of iron (say a cube one centimeter on each side) weighs more than the same cube if it were made of wood; there's more "stuff" inside the cube of iron than in the cube of wood because its atoms are mostly heavier but take up about the same amount of space. If something is denser than water, in general, it will sink. If it is less dense than water, it will float. Cooking oils are less dense than water, which is why they sit on the top of pots of water.
Ships are often made of wood, some kinds of which are less dense than water, and some of which are more dense. Ships can also be made of metals like steel (denser than water) or tough plastics (usually denser than water). You would imagine that a boat made of dense stuff would sink, but the boat floats primarily because of its shape. Boats stay afloat with heavy loads because they're hollow; they aren't solid hunks of wood or metal. This means that the boat experiences a really strong "buoyant force," upward, against the pull of gravity which is downward. You can see this for yourself if you put a plastic bowl upright in a pot of water or a bathtub. If you set the bowl in the water, it will float, but if you fill the bowl with water, it may sink to the bottom (depending on if the plastic is denser than the water or not).
The other factor that determines the buoyancy of a boat is the salinity (the salt content) of the water. Saltier water is denser than fresher water, so denser things can float in saltier water.
I hope that answers your question!
(published on 03/30/11)