Once did an underwater helicopter escape course. Designed for oil-rig workers so mainly aimed at passengers. It was good fun and suprisingly easy... in a swimming pool with divers checking you're ok. For good measure they also stuck us in another helicopter mockup and filled it with smoke before practicing evacuation.
The real trick they taught us was to sit with one hand holding onto something near your seatbelt release and the other stretched away from you, holding a bit of the aircraft, but pointing in the direction of the nearest exit. That way, when they dropped you in the drink then rolled you upside down you didn't get disorientated and you still knew which way to go to get out.
In some ways not very representative of reality doing it in such controlled conditions. It certainly convinced me that ditching was a pretty ropey idea. They had some impressive stats on survival rates for people who'd done the course, though!