Cayman,
One thing that I discovered just yesterday is that it often pays to slightly under-expose the shots (say -0.5 or so, sometimes even more like -0.7 to -1). This works really well with digital cameras at least, since it seems to help compensate for the "wash-out" effect of all the green light at dept. I suspect that the green-light tricks the exposure meters somehow. Under-exposing also works really well in the shallows during mid-day light conditions, since the rays on the rocks, coral etc. pick up some contrast and end up looking a lot more realistic.
When I post-process the pictures, I tend to reduce the amount of green (on the software I'm using, this is labeled "purple-tint" since it is a move towards magenta on the green-magenta axis). I also tweak the gamma a little in order to improve contrast and then fiddle with the brightness to compensate for the gamma adjustments (i.e. I try to keep the picture from getting any saturated regions, which look like fuzzy little white regions on the picture, and I try to make sure that the picture is bright w/o having an unnatural glow). Still, a lot of the pictures that I have posted have some regions that are over-exposed and a few of the picture glow a bit. Slightly under-exposing the picture seems to help with these two problems.
I suspect that a magenta filter (kind of like a Sea-View mask) might be a handy tool, since post-processing the pictures reduces the quality quite a bit (the better the picture starts out, the better it ends up).
Using artificial light is a big benefit. If you look at Alastair's photos, he seems to use flash a lot.
This photo (a lucky shot of mine) was taken using a flash and had absolutely no post-processing. Some of the really best photos in the gallery use artificial light as well.
The one other tool that I'm eager to add to my tool-set is a way to get wide-angle shots. Most of my best photos seem to be shot at the widest possible angle, with the distance between the camera and the subject being the used to set the size of the subject in the shot (not always possible!). This makes the backgrounds in the shots a lot more panoramic, and it makes the water seem much clearer.
Actually, I was going to post a similar question, but specifically targeted towards the Olympus C-5050, which allows you to set up a few favorite pre-set modes. I wanted to see if anyone is using that feature to good efffect. Hopefully Stephan one of the photo-heavyweights on DB will weigh in.
Can't wait to see your gallery...