After reading through tghis lengthy thread, I believe there may be one more scenario that has been over looked.
Is it possible that:
1) He was inattentive due to the fact that he was packing a camera and possible narcosis
2) He realized late in the dive that he was low on air and started for the surface. Once on his deco stop he finally sucked the bottle dry. To conserve air, he did not fully inflate his BC and maintained bouyancy by slowly swimming or hanging onto one of the mooring lines. He was controlling his breathing to conserve air, so it may not have been that apparent that he was there from the surface.
3) As he reached the surface he was OOA and slightly negative.
4) At the moment he reached the surface, he was struck by the departing dive boat. Not hard enough to cause injury or show outward signs of trauma, but enough to cause him to ingest water into his lungs.
5) Now he is OOA, negative, drowning and slowly sinking back to the bottom.
Not to lay blame on the dive boat and crew, but they should not have left with out everyone accounted for.....