There was NEVER going to be a smooth landing, with poor control of the aircraft, in gale force winds and rough seas, flying with virtually no visuals at cruise speed, I'm guessing in excess of 150 mph.
If he was wearing a cross chest seat belt, the impact at that speed would likely rupture his aorta, which would result in instant death. That would be the 'died of trunk injuries'. The head injuries, could be as a result of impact with something (including the pilot), or something similar to a 'shaking baby syndrome'.
He would NOT have drowned or suffered following those injuries.