It seems I'm going to be the first to break the "only one photo" rule. Sorry Alex. I tried to only pick one, but I have two favs for two very different reasons.
The first is of a hind that was hunting damselfish. We were diving on the house reef (Calabas) very near the end of our three week trip to Bonaire and had been captivated by all the animal behaviour we were seeing. I was watching this hind slink over and around the corals and rocks trying to sneak up on a damsel. I immediately thought it would make a good subject, if I could get the behaviour and if I could get a good background. Well, the hind slinked over the coral in the photo and paused as it looked at a damsel just out of the frame. That was the moment I caught. Very cool fish. I used a Canon 30D in an Ikelite housing, a 100/2.8 macro and a single DS-160.. and a little patience.

The other image, I shot yesterday at Whytecliff Marine Park, West Vancouver, BC (its about a 45-50 minute drive from my house). At the end of the dive, on one of those very rare sunny winter days here, I looked up at the surface. The vis and the sun were in agreement - finally! The image for me says something about why I dive, why I try to shoot and dive. I used the 30D, with a Sigma 15 fish and had a single DS-160 mounted, but turned off.

Sorry about breaking the one photo rule - I know I don't have any clout around here to be doing that.
Thanks.
Lee