Five years ago I back rolled off the boat for a negative entry drift dive with my air off. We had been having an incredible week of lobster hunting on that boat all week, so there was a lot of commotion and goofing around. I allowed myself to become distracted and complacent. I had geared up on my second tank during the surface interval, checked connections and tank pressure then shut the valve off as usual. What I didn’t do was PURGE the residual air out of the system. Moron! However, I DID remove all the air from my wing for the next hot drop. The captain finally yells dive, dive, dive! Glancing down at my SPG I see 3600 psi, so I hit the water with my heavy steel HP120, steel backplate and my typically empty wing. Very negative. Two breaths in there was zip coming out of my reg and I was dropping like the proverbial dirt dart. There was no panic, only frustration at knowing I would be abused for my stupidity post dive. I simply placed my left hand under my tank and pushed upward, then raised my right arm over my shoulder and grabbed the valve to turn it on, all while finning towards the surface. By the time my head broke the surface my air was on and I was ready to continue with my hunt.
Moral of the story, as stated so many times before on this thread, take a couple of breaths off of your reg while checking your SPG to assure your air is on BEFORE you dive.
Oh, and learn from your mistakes! I have never done this since. I also let the crew know I don’t require any help with my gear before they get anywhere near it.