One more thing... I've never ditched in a helicopter but did swim out of a near-shore ditch once in a GA fixed wing on a photo shoot. This was tropical, warm water. Egress wasn't hard, just regular seatbelts. We had to wait for the tail to sink enough to use the rear door (DeHavilland Otter), the pilot and co-pilot exited through their doors or windows.
As a non-pilot "civilian," even someone with many hours in light planes and helicopters doing photo missions, I remember there was a moment of disbelief -- "is this really happening??" -- when the plane hit the water. It was all very surreal. It took a few seconds to gather my thoughts, and get to work on exiting the plane. And this was in warm water.
I can't imagine trying to exit while inverted in shocking-cold water, strapped into a harness I wasn't familiar with, together with that initial reaction of not quite believing it's happening. Tourist passengers don't train for this. And these were young people in apparently good shape. I'm not surprised they didn't make it out.