I will try to keep it simple here, but:

A boat floats, because the fluid in which it is floating offsets the downward pull of gravity and pushes it up. The scientific name for this force, which allows even immense objects to float in liquid, is known as buoyancy.

Archimedes principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. The buoyancy of an object is related to its density. The density of an object is defined as, its mass divided by its volume.

If the total area of an object makes contact with the water is large enough, the object floats. The object must make room for its own volume by pushing aside, or displacing, an equivalent volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting a upward force on the object. Mercury for instance has greater density thus gives objects greater buoyancy. Objects of lesser density float on liquids of greater density.

This explains why wood and styrofoam float on water, whereas concrete and steel sink. It also explains why it is possible to make boats out of steel or even concrete. As long as there are portions of the boat below the surface of the water (that are hollow) the effective density of the boat can be less than that of water even though the real density of the material it is made of is greater.

Most of the space in the boat is taken up by air. The air makes the boat less dense than water.

I hope this helps,

-J