I was caught briefly in a strong downcurrent in Cozumel years ago. It was quite a surprise, and a good lesson learned.
Me and my wife were diving some wall (sorry, don't remember). With Living Underwater and Jeremy. We were at about 80 feet, just doing the usual stuff, watching the wall and drifting along easy. My wife (and buddy) was about 5 feet away, Jeremy was probably another 5 feet beyond her.
The first thing I noticed was I got cold. OK, no big deal, a cold spot. Then, suddenly I was surrounded by lots of bubbles (enough to be disorienting) and the bubbles appeared to be rising around me, like there was a diver or two right below me (there may have been, I glanced down but didn't see anyone, I was starting to feel disoriented). Then, quickly, intense pain in my ears. Hmmm. I glanced at my gauge, it showed 100' then I looked around. My wife was still a few feet away, now staring at me with a confused look on her face (and also surrounded by bubbles), and she was pointing to her ears. Yeah, I thought, that's weird, my ears hurt too. I noticed the wall appeared to be going up in front of us. I was just putting 2+2 together (although it was dawning on me more slowly than it should have) when I felt a firm tug on my BCD. Jeremy had grabbed me, I turned to look at him, he had my wife by her BC in his other hand. Our eyes met, I could read his expression (hey, idiot...). We all started finning - no extra air added to our BCDs - and we quickly popped out of it, around 115 feet. Released by the downcurrent (and then released by Jeremy) we all checked ourselves and gave the OK, and started finning up slowly, leveled off again about 80 feet and continued the dive (warily).
It was an odd experience. This was the only time I can ever remember while diving when I felt completely out of control, if only for a moment - we were definitely just along for the ride until we popped out of it. The downward surge was extremely powerful (obviously, in retrospect), one of those moments when you realize how big and powerful natural forces are, and how small and powerless you are by comparison. My impression of the experience was primarily of the odd sensation of being in dense bubbles rising up from below (at least I thought they were rising - I may have just been sinking but the hydrodynamics didn't make sense at the time). The most obvious manifestation was the sudden and intense need to equalize when I hadn't expected to - that was the unmistakeable giveaway.
I think I was in the process of understanding what was happening, and was about to take corrective action when Jeremy snapped me to attention. But I can easily see how one could get caught in one of these and be distracted by the changing conditions - cold water, disoriented by the bubbles and odd sensations, and I can imagine that one might focus first on the pain and the need to equalize - if one remained distracted for very long, you could easily find yourself very deep very quickly. I was glad we were diving with an experienced, savvy, trustworthy DM that day.
I like to think I would know better next time. I've been in relatively minor downcurrents since then (Indonesia) and was ready to swim away at an angle and/or add some air to my BCD, but never have run into anything as strong since.
Being aware of your surroundings and what's going on around you is always important. A basic truth re-confirmed that day.
