I have also been on an underwater Helicopter/Light aircraft excape and survival course at the same place. Its a one day course and it taught me that if you ditch and end up underwater isnide the cabin you will get out if you are alone and following the course instructions
I wonder if this is really true for a typical UNpressurised light aircraft.
The wing and tail boom cavities will rapidly fill up with water if the relevant part is submerged. All you have left are the tanks (hopefully partially empty!) and the cockpit itself.
If the cockpit fills up with water, would the tank buoancy be enough to keep it from sinking? I doubt it; my plane is ~900kg empty and the tank volume is about 300 litres (=300kg of water) so even if the tanks were full of air it would make no difference once the cockpit is filled.
So, if you are still inside the cockpit AND you are underwater, the aircraft will already be sinking to the bottom, and pretty rapidly too - ships are known to sink at up to 40mph and even a fraction of that is likely to be most uncomfortable.
If the water is only say 10m deep and is very clear, it would make sense to wait till movement stops and then try to escape, but most of the sea is far deeper than that, and very dirty.
So the idea must be to get out immediately the aircraft settles on the water. Especially if it is a Cessna (no wing flotation).
I have also read that a drysuit could be quite a hindrance if one is submerged - a bit like an inflated life jacket. I know one can let the excess air out by poking a finger down the nect/arm seals but someone in this situation won't have time for that.
Have I missed something obvious?