Why do big boats float?
For grown-ups: the science
Big boats float because their hollow shape pushes aside water, and the water pushes up on them.
A floating object displaces water until the buoyant force, equal to the weight of the displaced water, balances its own weight. A boat's large hollow hull gives it a low average density, allowing it to displace enough water before it sinks.
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Transcript
How can a big, heavy boat stay on top of water?
A boat has a wide, hollow bottom that pushes water out of its way. The water pushes up on the boat.
The boat pushes away enough water to hold up its heavy body. That upward push keeps the boat floating.
Air fills much of the boat's hollow space, so it is not solid all the way through. This helps the boat stay above water.
So that's why big boats float! Their hollow shape lets water push up and hold them.