Would it not be possible to have a central release system to the harnesses that was activated by water? From what I have seen, 5 people should be around to tell their stories of this accident. If you can't escape a sinking helicopter that has no doors, you have either been knocked unconscious (as mentioned above) or you are attached to it and can't free yourself. Does dunker training (which we can assume they haven't done) cover using a knife underwater? Assuming you can find it whilst suffering cold water shock in the dark or with your eyes closed, can you hold it, can it float away, does it easily cut through wet harnesses? Thinking about trying to do that makes me shudder. A fun day out suddenly goes extremely wrong. You have the shock of knowing the machine is going down so probably don't instantly decide to grab the knife or look at your harness. You then have the impact shock and the reality. Then the water. Then the panic. If the harness hooks could automatically be freed, you would surely have a chance?
I have flown over water and the pilot briefed us that in the event of a forced ditching, he would instruct us to remove our belts before impact - assuming it was a controlled auto, then brace.
Shocking to watch and I feel sorry for the pilot whatever he may or may not have done prior to the flight. To be able to swim away leaving the pax behind will live with him forever.