I'm not quite Rescue diver yet, have it planned in the next couple of months so I can continue on with my Divemaster. However, I have been diving the Caribbean for years and I have seen many near-misses. In Cozumel, in about 90 feet this lady's tank slipped down, the tank was hanging by her regulator in her mouth. The DM had just looked around with the OK sign when it happened so he had just turned back around. I saw this and immediately started swimming towards her. Her buddy was oblivious, he was only OW certified and not really "aware". Anyway, when I got to her, she had taken her BC completely off! In 90 feet of water! I grabbed the tank and her, almost did a mask to mask visual to get her to just relax, and I got her fitted back up before the DM could get there. You really can get a LOT of expression out of people's eyes - both this woman, who realized then that what she had done was not smart at all, and the DM who ended up buying me a dinner that night. I truly believe I was watching a diving death in front of me, and I decided not to let it happen.
I have a couple of other less dramatic incidents, but this is why I decided to complete my Divemaster training..... just to know more. After this was all over I had a bout of nerves, although it was strange how calm I was, it was like in slow motion.