One substance floats on the surface a fluid when density of the substance is less than that of the fluid. When someone drowns and dies, the body starts to sink because the air in its lungs is replaced slowly with water thus increasing density. Once the body is submerged, its decomposition starts.
It remain submerged for some days until the bacteria in the gut and chest cavity produce enough gas(i.e,methane, hydrogen sulfide, and carbon dioxide). As the gas is light the body float back to surface of water.
Not all parts of the dead body inflates the same amount. Abdomen and chest bloats more as it contains the most bacteria and cavities, thus rises first followed by limb and head. Since limbs and the head can only hang forward from the body, cadavers tend to rotate such that the torso floats facedown, with arms and legs hanging beneath it. Most dead bodies float this way, that is face down, but there are exceptions. The smaller the limbs, the more likely a corpse will float facing up—short arms and legs create less drag.
Also, if a body stays on the surface of the water for a long time it will release the built-up gas and sink once again. Decomposition continues underwater—more gas accumulates—and the body may become what rescue workers sometimes call a "refloat." Since refloats are at a more advanced state of decay, they may be more evenly bloated and thus more likely to float faceup.https://www.quora.com/Why-does-a-dead-b ... in-water-1
...Decomposition gas that can't get out of the body, makes it great buoy.
Decomposing bodies have pulled up entire outboard motors, with chains attached. They're that buoyant.
< Jeg tror det er den fejl han har lavet