In the instance under discussion I imagine something that he couldn't handle overcame the hapless pilot.
In general you guys have commented on radio aids for this sort of water crossing. Where one possibly can I personally look out for and would try to ditch in front of a likely slow looking vessel.
[Same technique as keeping potential forced landing fields in sight when aviating over land.]
That way someone else may see & report it as well as try a rescue.
mike hallam.