A couple years ago I learned how to SCUBA dive. I had a hard time and had to come in for extra pool sessions to keep up. Anyway, it was a friendly setting and I was there voluntarily and there were a few times when I thought I was drowning. It was terrifying. Sometimes I had to leave class early, I was so scared. A good lesson was the one where I never felt that panic. Most of the time, the best I could say was "Well, I didn't die." And I was in control, you know? If I wanted to ditch, I could. But I still, every now and then, have a drowning nightmare.
I can only imagine what it would be like to be a in a situation where I thought I was drowning AND the environment was not something as safe and happy and controlled as a SCUBA class. Or if the environment was just outright hostile and the people making me think I was drowning were people who wanted to hurt me and I had no choice (as opposed to SCUBA class, where they didn't want me to think I was drowning and the whole point was to be safe).