Q: Why do dead bodies float in the water?

Krishna: That depends. 

Dead bodies in the water usually tend to sink at first, but later they tend to float. The reason? 

The average living body has a specific gravity very close to that of water. Buoyancy  (the ability of an object to float in water or air) is intimately related to specific gravity. If a substance has specific gravity less than that of a fluid, it will float on that fluid. 

A body's density is how heavy it is compared to its volume. A living person's density depends on factors like the amount of air in their lungs and the amount of fat on their body. When someone dies, the air leaves their lungs, making them denser than water and causing them to sink. 

After death, even small variations in floatability, like air caught in clothing, heavy clothing, things in the pocket of the person in the water or on the body can affect whether a body sinks right away.

Once the body sinks and goes to the bottom, its own enzymes and internal microorganisms, especially anaerobic ones in the intestinal tract, gradually break down the soft tissues into liquids, salts and gases, including hydrogen sulfide, methane, carbon dioxide, ammonia and hydrogen.

A dead body floats on water because its density decreases after death, making it lighter than water.

Heavy clothing and purposely added weights may increase the time a corpse spends immersed, but will not usually prevent it from rising eventually.

Putrefaction is slower in water than in air. It is slower in seawater than in fresh water, and slower in running water than in stagnant water.

As the body decays, its volume increases, making it lighter than water.