Why the dead human sink then float?

Answer 1

Depending on the mechanism of death the air can be either missing via crushing or collapse, or replaced via drowning. The rest of the average body under normal conditions is made up mostly of water with slightly higher density over all, hence sinking.

But the body is home to a large amount of independent life, most obviously in the digestive system. When the immune system is compromised as a result of death, these bacteria begin to consume the body as they would normally do with food you eat. This process releases gas as a by-product of metabolism.

Normally this is limited in extent or expelled, but upon death the digestive tract ceases function, these gases build, bacterial populations explode, thus creating pockets that increase the overall buoyancy of the body, and since the body was mostly water to begin with it doesn't take much of a change to cause it to rise.

As an experiment float in a pool and take a deep breath and then let it out slowly paying attention to your sinking a little upon exhalation and rising as your lungs inflate.