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 According to the Archimedes' principle, when a body is partly or fully immersed in water, it displaces water equal to its volume. A body floats on water when its weight is less than the displaced volume of water. If its weight is the same as that of the displaced volume of water it will remain fairly steady, at whatever depth it might be. Usually the bodies whose density is less than that of water, float. For example, wood or cork float on water because their densities are less than that of water. Bodies whose density is more than that of water, sink.

 The density of the human body is less than that of water. Therefore, when a man falls into the water, he floats for a few seconds in it. But when water goes into his body, his density becomes more than the water and he drowns. When his body starts expanding due to this water, the volume of the body increases and thus the density decreases. In other words, the weight of the water displaced by his body. And as a result it starts floating.