I suppose this doesn't necessarily rule out that he died 'innocently' and then his body was shot at, but it does make that less likely. He could have died in the crash before he could drown - such as the force of the car hitting the water maybe leading to a death that might not show up on autopsy, like some kind of internal decapitation (neck vein severed by the force) later being covered up by the bullets. But if he truly didn't drown (I've read that can be harder to ascertain than we think) that does bring up more questions of what happened between him and Michael or the both of them and someone else. I see that Michael was found but not sure about the car. If there was a gun in the car or obvious bullet holes or a mechanical reason for the crash, that might be a good clue.
I do hope LE are asking these questions and aren't just closing the case now that he's identified.