Having helped retrieve said deceased persons from lakes and rivers, we find that, generally, in the warmer months (at least here in the western U.S.) that a person who drowned will sink as soon as the lungs are filled with water. They tend to sink nearly to the bottom, but, depending on clothing, fattines and other factors, they sometimes float sveral feet above the bottom. Decomposition begins almost immediately, but is dependent primarily on the water temperature. If the bodyof water is 50 degrees F or better, the body will float to the surface within about 5 to 8 days. the floating is caused by the decomposition gases trapped in various places in the body.
Although there is bloating and discoloration, very little loss of lesh actually occurs. If the body floats without being found for 2 or 3 days, sometimes the gas escapes from the body and it resinks. It may never refloat, but can as the decomposition continues.
As one poster mentioned, fish can play havoc with a dead body, but unless it's in theocean where larger fish can work on it, this takes a lot of time.
We've retrieved snowmachine riders who fell through the ice in the dead (no pun intended) of winter only if there was no current in the body of water and find them in the immediate vicinity. If they move away due to the current they are often not retrieved until spring when the water warms and decomposition begins in ernest (or Ernestine, as the case may be)...