I remember having that experience once, when I was 17. DA and I were on my first big trip (Boston, New York, DC, and Bermuda) and she must have thought it would be nice to give diving a try. So we signed up for a dive, and on the boat out to the site we were given paperwork to fill out - pages and pages of release forms with about every way to say they weren't responsible for any of our deaths, and that people with asthma might have some trouble (and I have asthma). But the sun was shining and the water was blue, and the whole thing seemed like a good thing to do.
After the paperwork came the instruction regarding the tanks and how to breathe and decompress. So down we all went (ten of us, maybe a dozen). Our instructor had taken us to the site of a couple of ship wrecks, so there were bottles and other assorted junk on the bottom (which was about 30' down), as well as very colorful and curious fish, which captivated me much more. (Anyway, I'm sure the bottles were planted, even though they did look like they'd been there awhile.) The fish would swim up to your face and stare, then dart off sideways with their sinuous and flexible bodies, in the way that fish do. I was mesmerized. Not to mention it was so nice to be in a bathing suit in warm water--no wetsuit. Not that I'd ever tried one on, but I'm claustrophobic, and I'm sure the suit would have driven me nuts.
So fascinated was I that there came a point when I realised that I was alone. Everyone else was nowhere to be seen. Perplexed, I surfaced and took a look above water, but strangely enough I could only see a little fishing boat, and when I asked them, they said they hadn't seen a group of divers. With this unsettling news, I went down again, and this time, a few moments later the instructor appeared again. We swam a little more, he showed me a few more "artifacts," and then we went back to the boat, where everyone else was waiting. While I was otherwise engaged under the water, everyone else had returned to the boat. Why the fishermen didn't know what I was talking about, or claimed not to have seen the other boat is still beyond me. But, I enjoyed the whole experience, and wonder if some day I might do it again.
More water thoughts later...