I have done this in the army a couple of years ago, you have to do it every two years to keep the qualification in date. Mine was the same as this but under cover. You do it four times. The first one the "helicopter" lands in the water and gently sinks but stays upright, you are in the brace position as it hits the water, you then grab the seatbelt buckle with one hand and the other grabs the edge of the window. The person sitting next to it punches it out, it's only plexi glass. You then go out the window in turn, not a big free for all. If you lose the grip on the edge of the window you have lost your reference point. The second attempt the helicopter hit's the water and rolls upside down, you wait until it is properly upside down and stopped before you go out the window, as it rolls in real life rotor blades will be snapping as they hit the water. The third attempt is in semi darkness and the fourth attempt is in full darkness, both upside down.
In real life the water maybe be freezing leading to a cold water shock, the cabin would have all sorts of things floating round in it to make it a lot harder. From the video the airframe looks like a Chinook, I did mine in a Sea King.