My knowledge is based entirely on TV shows, but I thought people who drowned had water in their lungs. If they're already dead, their lungs don't fill with water as they do if you die by drowning.
If the person was intentionally drowned, however, they would have water in their lungs. I guess the coroner would have to check for various things (bodily signs of a struggle and so on) to make a determination if it was an accidental drowning or a homicide by drowning. If the person couldn't swim and was just tossed in (rather then being held down while they struggle) and drowned, I am not sure how a coroner could tell if it was an accident. (I am not suggesting that Deorr's parents drowned him)