Apparently, the victims didn't drown, they got caught up in the conveyor and were dragged under the stuck raft and crushed by the weight of the raft.
Since it took some time for the raft to rise up, tip them out, drag them under and then mangle them up - simply pressing the emergency stop button very likely would have save all their lives.
It seems the operator didn't know that the bright red mushroom shaped button with "Emergency Stop" written on it was the right button to push. “She was told not to worry about that button, no one uses it?”
If that is how competent the ride operators are at Dreamworld, the rides are about as safe as getting on a worn out bus with a driver who don't know the difference between the brake and the accelerator. Is it the driver's fault - or the employer's fault for not ensuring that the driver can do his job?
