Corpses that have a watery grave will begin to float within a week's time. Here's why:
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.
A corpse begins to sink as the air in its lungs is replaced by water.
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.
And 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. As these gasses accumulate in the body's cavities, the corpse begins to rise to the surface of the water.
"Bodies in Water" (New York Times)
"Last Word" (New Scientist)