I did this training when I was a cadet in Air Training Corps. It petrified me, I did the standard dunk but bailed before the dunk and inversion attempt. I am not a strong swimmer, they had rescue divers there, no way.

Being strapped into a container slowly filling with water rushing in through various orifices is a horrible feeling. And having to wait for the water to stop rushing in before unstrapping.

And I was in an indoor pool. Those poor people.