The density of an object is a major factor that decides if an object float (like a basketball) or sinks ( like or stone) . All items that have an average density less than water will float. Since the stone 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. Boat, 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 hulls 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.