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>Observation 1: latex is heavier than air. Balloons are made of latex.
>Observation 2: helium is lighter than air.
>Test: Fill a balloon with helium.
Given enough displacement and assuming no leaks, despite being made of a material heavier than air, the balloon soon begins to floats upward. Fill the same balloon with air, and it sinks.
Same principle.
>Observation 1: metal is heavier than water. Boats are made of metal (in this example).
>Observation 2: air is lighter than water.
>Test: fill a boat with air.
Given enough displacement and assuming no leaks, the boat will float. Fill the same boat with water, and it sinks.
Unlike a balloon's helium, a boat's air has nowhere else to go but along the surface of the water where the boat (ideally) already is. For this and other reasons, keeping the air pressurized is not needed.
