Game officials say it was just before noon when they received a report of an unmanned motorboat running in circles.
A game warden was able to jump aboard and stop the motorboat.
A few moments later, a member of Belgrade's Fire Department found the body of 64 year old George Bonney.
He was located about a quarter mile away from the boat, floating on the surface of the water.
Game officials say he was not wearing a life jacket and was the only person aboard the boat when something caused him to enter the water.
The Medical Examiner's Office is now trying to determine a cause of death and wardens are trying to figure out what caused him to go overboard.