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Anything that was designed to float, was based back to a theory made a very very long time ago, by a man called Archimedes. His principle, clearly named Archimede's Principle, explains how things float.

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Whenever you place weight into a tub or bucket full of water, the water rises or could even overflow. This is because you displaced the water with the weight of the object you placed and it had to go somewhere. The important thing to float is that the object must displace an amount of water which is equal to it's own weight.
For example, say you made a plastic boat. The boat weighs 20lbs. You lower the boat into the water, and it'll go down until it has displaced 20lbs of the water. That means that 20lbs are pushing up on the boat and making it float.
Your boat needs stability. Stability means that it's designed not to tip over easily.
The more weight and larger the boat is, the easier it is to walk and keep your center of gravity stable. Like an aircraft carrier, it can hold massive amounts of weight and is very large itself. But a row boat is much lighter and just walking on one side can cause the boat to tip over.

More information on boats and how things float:

http://boatsafe.com/kids/021598kidsques.htmexternal image moz-screenshot.png
http://www.grc.nasa.gov/WWW/K-12/WindTunnel/Activities/buoy_Archimedes.html

For more information see the page in this wiki on Archamedes principle.