There are about a hundred things that would cause this tragedy. To answer this at the moment would be speculating.
Maybe they lost an engine? Maybe the parameters were placed in the Garmin incorrectly? Maybe no takeoff checklist was performed...
Wheter you dunk it in the Tasman, South Pacific, Atlantic or Gulf of America (LOL) salt water kills. You had better flush it with distilled water 30-60 seconds after it hits the water. Fresh water dives are a little different but they still...