Short version is that I passed out about 90 seconds after entering the water. Fortunately I’d just told my buddy I wasn’t feeling well and wanted to abort the dive so he reacted immediately, lifted me to the surface & got back to the waters edge where I was unconscious, blue, not breathing & had no detectable pulse. I was at an inland site where another group had just finished doing some rescue training so they were on me like a shot, and the site’s rescue boat was over about 30 seconds later. I came to with an O2 mask on my face while they were doing CPR on me, then was taken to hospital by ambulance & discharged later that night.
At the time I put it down to complacency and assumed that I’d effectively been breathing a gas mix that was only 10% oxygen and so had passed out as a result, but about 9 months later I had a migraine type attack that left me curled up in a ball, gasping for breath and twitching and then realised that a lot of the symptoms and after effects of that were the same as when I had my accident. I’d never had that before or since, but it seems the more likely explanation now.
Don’t let that put you off though! ‘Normal’ diving is pretty safe these days and the training will teach you how to react in the unlikely event that anything goes wrong. If you’re comfortable flying downhill on a bike then the underwater world has way less hazards than that! ;-)