I know they can do remarkable things with a post mortem, but I don't think they'll be able to do much more than determine she died by drowning and has been in the water for three weeks. Maybe they can tell if she had any alcohol in her system, although that's likely also been blown way out of proportion with people assuming she's been driving around while absolutely bladdered, etc. Or they may be able to tell she had some kind of seizure, before or after hitting the water. I think it'll be unexplained death, no third party involvement, by drowning. End of story.