Cave photography and lighting pools of water.
I've posted a few photos from my cave trip in another thread here: http://www.dpreview.com/forums/thread/3668637 The examples you can see there were all taken with natural lighting (plus headlamps), and in general I found that the natural lighting often worked out far better than I had expected.
If you click through to the Facebook album you'll also see lots of examples where light painting was used, and a few with flash. The flash units I took with me didn't prove to be that useful. This was primarily because there wasn't usually enough time available to make good use of them. Getting the flash units and radio triggers out of my bag and configured took far longer than using a torch to light paint. Additionally, I found working with flashes in sometimes complete darkness to be far more challenging and less predictable than the light painting approach, they were hard to position correctly without a fair bit of trial and error which often meant clambering over rocks for a fair distance each time. The flashes came into their own however on a couple of the river crossings where I could freeze the action - it wasn't very practical to get people to stand still in that situation and I could get people to handhold the flashes off to the side.
I tried a few shots with underwater lighting but for the most part I found the results to be either too uneven or too unnatural looking. I preferred to put a torch underwater instead of use flashes since it was much easier to know in advance what sort of results to expect that way but even so I didn't feel I really got it right in the limited time I had available. I'm sure if I'd had more time I could have obtained much better results so it's something I'll definitely want to try again when I next get the chance.
One other semi-lighting related comment: I had a small laser pointer and it proved to be extremely useful for indicating to people where to position themselves, what rock to stand on, when to stand still or relax, etc. We also had radios that were great for passing on more detailed instructions to helpers and models. Without the laser and radios I don't think I'd have been able to get many of the shots that I did. Also, it was a lot of fun in the evenings for everyone to take turns drawing rude things on the cave walls while I took long exposures of their efforts!
Thanks DukeCC for your tips, much appreciated!