When I raised this point yesterday in the "When you want shallow DOF" discussion (dSLR/housing forum) I thought perhaps this was better raised in a separate tread in order not to hijack the original discussion. So thanks for doing just that.
For me composition rules try to capture subliminal preferences of human taste and they can be very helpful to understand why certain images are more appealing (to many) then others. However, if you just apply the rules it takes all the creativity and spontaneity out of it. Knowing when you can break a rule and get away with it shows true mastership but then you really have to have a good knowledge or intuition for the logic behind the rules.
Eric Cheng posted an interesting series of thumbnails yesterday from a recent dive with annotations on why he chose and changed certain compositions. Some of these involve divers (no lights this time). Near the top a diver is swimming far in the background without distracting from the main eye catcher, an imposing red seafan. For me this shot worked as the diver didn't interfere with the image and added depth.
Near the bottom Eric lamented that he wanted a diver next to a large barrel sponge to give a sense of scale to indicate really how big the sponge was. Another logical use of a diver (although opening a can of nassau groupers and let them swim next to the sponge would be better).
A bit higher up, a hollow barrel sponge has fallen over and Eric suggested that it would have been nice to have a diver swim behind the sponge and appear in the hole of the sponge. You would have to see the result to judge if it worked, but I expect it would have looked like an over-engineered scene that just doesn't feel real. The two little fish instead make a beautiful and very natural composition.
One good thing about wetpixel is that you get to look at a lot of great images which I try to use to understand what they did to make it such a great image. The next step is to take this under water and try to sense a good picture before it has actually been shot. I've still got a long way to go but as long as I can learn by looking at great pictures I'm a happy student.
Bart