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Our body density is higher than density of water. So we generally sink in water. Muscle cells and bone cells are highly dense. So skinny and muscular people will easily sink in water. Fat is of lower density. So fat people tend to float. It sounds a bit ironical, but true.
 Cells in our body contains an organelle called Proteosomes. It contains chemicals which are highly oxidizing. When cells die due to death of human, they rupture and the chemicals are released in the cells. It oxidizes most of the organelles in the cell and gaseous by-products are formed which makes the body lighter. Hence the body floats. 
It has nothing to do with intake of air or oxygen or whatever. 
Density of a human body is greater than that of water.its due to the weight of the skeleton system and minerals and nutrients of our body.when we dies our dead body floatsdue to the metabolic actions and chemical reactions happens in the body