As I've said before, if I go equipped for anything that might crop up, my photography is bland and uninspired. Diving with just the equipment for the shot that I've planned yields much better images for me.
That said, I dive with a wing and soft-pack harness to which I've added DiveRite pockets. I use a Gorillapod for an external strobe sometimes, which is clipped onto the bottom D-ring on the side of my harness (on the other side is my reel, on every dive). One Pocket has a fold-up snorkel, a wrapped-up buoy and a compass on a lanyard, the other can have photographic bits, except that they're usually on my camera. I've bought a threaded holder for wet diopters that is velcro'ed to the float on on strobe arm.
Techies are good at loading their harnesses without impeding access or their movement, and I've learned a lot about rigging gear from a Tech diving course. Putting pockets over your hips and clipping gear behind your ass(US) or bum(UK) works pretty well.