Once when I did the Mountbatten course I was watching all the fixed-wing pilots being dragged along behind the boat, simulating their parachute. I was on choppers at the time, and was wearing a waistcoat dinghy. After I pointed out that I wouldn't be dragged, it was pointed out back that, unlike the others, I wouldn't have my life-vest inflated until clearing the aircraft, under water. That was followed by a hefty push off the back of the boat. I don't know how far down I went before I managed to inflate the life-jacket, but the bottom of Plymouth Sound was there somewhere.