A small iron nail sinks in water, but a heavy boat floats on its surface. How?

It is the density of an object that determines whether the object will sink or float in water. Objects with lesser density than water will float on the surface where as objects having density more than that of water will sink. As most of the space in the boast is taken up by air so it causes the overall density of boat to be lesser than that of water. Whereas, the density of the iron nail is more than that of water. Due to this the iron nail sinks while the boat floats on the surface of the water.