I took graduate level physics (which I have since totally forgot BTW) - so I am an authority on this matter. ;:;rofl
I think something you have failed to visualize Dean is that, because the boat folded back so significantly, the dash/seats/structure of the boat that the occupants got launched into
were also still moving forward as the occupants were initially making contact with them. The victims made contact about the time the first foot or two of the front of the boat folded back.... but as they made contact with the boat its front end was continuing to fold - allowing the rest of the boat to continue to move forward (allowing the dash, back of the front seats, etc. to decelerate less severely during occupant impact).
Still horribly jarring of course - but much less so than if they were in a stiffer stronger material that didn't crumple.
In other words they didn't slam into the dash/seats/people in front of them that were going 0 mph - they slammed into things that were still moving FORWARD
with them. That effectively caused them less impact damage - whether you want to accept it or not. How much is of course dependent on what they hit, their position, etc.... I just know I'd prefer to hit something going in the same direction as me, not sitting still.
Now would a fiberglass boat have "crumpled" any less? Maybe - maybe not - I bet it depends on the design/construction. But the boat spending time decelerating by "sacrificing itself" in the front is a good thing for occupants - seatbelts or no seatbelts.