Hey, I just got the diagonal bit from a recent, brief article by Stephen Frink in a Scuba Diving mag. I guess I'd found myself doing that, but never really understood why. If I remember right (didn't dig the mag up this a.m.) the rules he spoke of were:
1. rule of thirds (we all know this one)
2. negative space for things to move to
3. strong lines pointing toward diagonals
4. avoid shots that can be bisected either vertical or horizontal (kind of goes with rule of thirds)
5. ignore all rules if something looks appealing for some reason or another (sometimes rules just don't apply)
Here's an example of a not-so-stellar eel shot (missed the focus a bit, and too deep a DOF so it's really 'busy' instead of honing in on the subject more) that I still like because of the color and texture, and that I think was framed well because of that diagonal rule. The eel himself is just in the middle since he wasn't moving so I wasn't trying to imply motion...
I get what you mean about it taking a long time, though. It took me over a week of processing (with a lot of initial RAW development happening during the trip itself) before I got my shots up from my last trip, and many of them are up just because I was 'filling the set' ...like a really crap turtle shot because it was the best of a bum lot. Even that eel shot above isn't great, I know, but I still like it.
Those very few I found myself really liking, I did spend a lot more time on, and probably still aren't done. For one thing, I don't remember if there was a rule about cropping and aspect ratios (again I'd think that could be broken, like using tall thin crops for strongly vertical subjects you planned to arrange on a wall, say scenes from a kelp forest maybe), but I tend to try and keep my camera's 4:3 aspect ratio so I always crop to either 0.75 or 1.33, depending on orientation. Sort of constrains me sometimes, but it's just something I've gravitated to for now. And for another, I made a mistake of reading some postings by Alex Mustard on Wetpixel about how color balance effects the background blue, and now I can't help but see some of my blues look a bit 'muddy' not crystal clear. Granted some of that was probably the water, as vis in Dominica was more in the 40-60 range than the 70-100 gin-clear range, but still.....
BTW, I never did tell you - very nice shot. Those tiny buggers are a pain to get to sit still long enough (or to predict) for a shot. I've never gotten one at least.