I was reading this mystery novel last night, and in it this guy goes missing and then a week later they find his body floating in a nearby lake. I don't get it--I have a hard enough time staying afloat for ten minutes, and I'm alive. It's a mystery how corpses do it.
Let's see if we can figure this mystery out. The first clue is that the density of the human body is similar to the density of water, and what keeps us floating--other than the dog paddle--is the air in our lungs.
You'd think a corpse would sink as the air in its lungs is replaced by water. And they do--at least initially. But when any organism dies, it goes through putrefication, the series of chemical, physical, and biological changes that end up returning the body to the food chain.
When a body dies, the bacteria that normally live in the digestive system continue to feed on the proteins and sugar in the body's soft tissues, and to excrete gasses including carbon dioxide, ammonia, hydrogen, and methane.
These gasses accumulate in the body's cavities, and--if it's submerged in water--eventually cause it to start rising. Decomposition happens more slowly in water, and even more slowly in cold water.