I accept the pilot was 'busy' during the engine off but my personal take is that the excessive rate of decent caused the helo to hit the water too heavily allowing the mainframe to submerge temporarily thus allowing the rotors to impact the water. This must have aggravated any rotating movement of the frame flooding the cabin.
A Zero/Zero would possibly have allowed the cab to remain upright long enough for a relatively safe egress of pax. RiP