Why do Boats Float on Water - Do you know why some things float while others sink? It isn't just because of their size or their weight. If you can float when you go swimming, why does a small pebble sink? The answer is that floating or sinking depends on an object's density. If an object is more dense than water, it sinks. If it is less dense than water, it floats.
An object is more dense than water if it weighs more than the same amount, or volume, of water, Pebbles are made of gritty particles, which are very heavy and packed together tightly. This is why they sink. They are more dense than water.
Steel has a higher density than water, so why doesn't a steel boat sink? The answer is that the inside of a ship is hollow and contains air. It also contains other materials which are less dense than water. The density of the whole ship is less than that of water.