Archimedes Principle

A floating body must satisfy the following conditions:
i) the weight of floating body should be equal to the weight of fluid displaced by the immersed part of the body.
ii) the centre of gravity of the body and the centre of buoyancy of the liquid displaced, must either coincide with each other or lie on the same vertical line.
Ships are hollow from within and have fairly large volume. Thus, they are able to displace more weight of water than their own weight and float on the surface of water.
The weight of hydrogen filled balloon as well as its fabric is less than the weight of air displaced by it. Thus a net upthrust act on the balloon and hence it rises upward.

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