Very good question. It makes sense that a wood boat floats because we see wood logs and sticks floating all the time. Our common sense is baffled when we see a boat made out of steel or cement floating because we don't typically think of these items as being very buoyant.

It turns out this problem of floating doesn't depend directly on the type of material but rather how the material is used.

Floating objects are described by the Archimedes' Principle. Archimedes found that in order for an object to float, an object must be able to displace the amount of water equal to its own weight. If the object can't displace this amount of water, it will sink.

This theory of displacing water can be seen when you get into a bath tub and the water level rises. We know that a ship's shape is very crucial to its ability to float. So now we have said that floating not only depends on weight but also on shape or size. Since the floating depends on weight and size, we see that a ship's density is a good way of determining if it will float. Density is equal to the objects mass divided by the objects volume (density = mass/volume).

Water has a very high density. A cube of solid steel has even a higher density, hence why it doesn't float.

Because of a ship's design it has a unique density. A ship's density has to include all the weight in the entire volume of the ship. Therefore, this includes the center of the ship which is mostly empty, or air. So a steel ship has a smaller average density than water because of the empty center of the ship. By being less dense than water the ship floats.

Therefore, when determining if a ship is going to float it is important to analyze the density of the whole ship, not just the materials on the exterior. I hope this helps.