Boats float because their shape and weight allow them to displace the equivalent weight of water as the boat without allowing the water to overflow into the boat. Scientists use Archimedes' Principle to explain this phenomenon. According to HowStuffWorks, Archimedes stated that an object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object.
In order for the boat to stay afloat, when placing weight onboard, the user must distribute it evenly, especially in a small boat. When a passenger in a small boat stands up or moves to one side, the center of gravity of the boat shifts. If this shift is too large, the side of the boat tips below the surface of the water, and it begins filling with water. Every drop of water adds to the weight the boat must support. If too many drops accumulate in the boat, the weight of the boat and water become more than the boat can support, and it sinks.
On large ships, the weight of passengers on the boat is negligible. This allows passengers to move about freely on the floating city, taking part in recreational activities, dining, shopping and relaxing. The design of such ships includes very tall sides and tons of steel, but Archimedes' Principle keeps them afloat.