Why doesn't a ship sink? — Phil, Philadelphia, Pa.
In order for something to float, it needs to displace its weight in water. So if you have a canoe that weighs 100 pounds and you put it in water, it will sink into the water enough to displace 100 pounds of water. Because water is heavy, the canoe only sinks into the water a few inches to displace 100 pounds of water
A big ship made of steel is heavy. But most of the interior of the ship is air. The average weight of the steel plus the air is much less than the weight of an equivalent volume of water. So the ship sinks into the water several feet, but it floats easily. For more information, see this page.