I am reviving this thread as I just had an experience such as this this summer and would like to know what I can try to do to overcome this and to share my experience with other new divers and hope it helps them too.
We were in a quarry vis was not very good and I was slightly over-weighted. I can't seem to descend at all in my 7/7 farmer john unless I'm a little over-weighted, but that is a question for a different thread. My buddy and I had just descended and were looking to hit about 50-55 ft then follow the wall to something or another that we wanted to check out and everything seemed fine. Then I saw the paddlefish. I was so excited because they had been so elusive thus far, I rose to a vertical position (hovering) to get a better look and to get my buddy's attention so she could see and I started to sink.
OK, at first I was ok, I tried to put a few bursts of air into my BCD and kick but continued to sink. I hit a thermocline and the temperature dropped drastically and my throat and chest felt like they were constricting and I couldn't breathe. That is when I began to panic...and I was still sinking regardless of how hard I was kicking. I couldn't seem to get air into my lungs and I was beginning to hyperventilate. It didn't help that I was already at 67' and still going down and I thought that the max depth of where we were was 70' but I couldn't see the bottom anywhere near me.
I got enough air into my bcd to stop my descent and I did start kicking toward the surface. I was not shooting to the surface, but I was definitely swimming up faster than I should have been going. I did motion to my buddy I was surfacing as I swam by her, totally blew my safety stop. Even as I got into shallower water, I could not stop the hyperventilating. When I broke the surface, I was "with it" enough to fully inflate my bcd then I ripped the regulator out of my mouth so I could 'breathe'. I was in tears and I just wanted to get my hood off and I couldn't get it off and it felt like it was strangling me. I was a mess.
OK, so now my rational brain takes back over even as I still am having issues breathing. I realize that I'm really lucky I didn't throw an embolism and other logic start pouring in too...first, I'm in a quarry, I still had a relatively full tank of air and we weren't on the deep side of the wall, so the chances are I really had been pretty close to the bottom at 67'. Second, had I really been in serious trouble I should've dropped my weight belt. Third, if I could've just gotten myself back into a horizontal position it probably would've helped. Fourth, I really need a different hood, I really like diving with beanies but they're not so good for deeper dives.
My overall point is I consider myself a relatively logical person, but the panic did override the logic. I also learned from other experiences, that I have issues with cold water and low vis diving and I think that brought on the issues more so than the depth. I'm hoping more experience will help me overcome this, I'd really hate to think I'm going to end up having to be a resort-only diver (80f water, and 50+ft vis) because of these issues.
On the other hand, when I did my deep dive for my AOW cert after this incident, I felt the panic coming on again because the water temp was like 45f and I was having issues breathing again, but my prior 'panic' attack and then my self-analysis of the situation helped prevent the panic from overtaking me and keeping me in my 'logical zone'. I was able to get my breathing relatively under control until we got back to warmer water and completed the dive with no issues. No one else had even been aware of my 'almost' panic.
Sorry for the long post, but if anyone can suggest anything for overcoming the cold water and low vis issues, I'd appreciate it.