The density of an object is a major factor that decides if an object floats (like a basketball) or sinks (like a stone). All items that have an average density less than water will float. Since the stone is thicker and denser than the air-filled basketball, the stone will sink. The basketball being less dense means that it will float. Boats,whether they are a tiny sailboat or even a very large oil tanker,all have hollow hulls. A hull is the bottom section of the boat or ship's frame. The hollow hull are filled with air. This makes them less dense than water. Hence,the effect of having an air-filled bottom section to a ship means that it will float.