OK, plenty has been said here, and I don't want to rehash it. But as a fat American photographer, here's a tip for those of you who might get stuck with one of us someday:
The most important thing to note before agreeing to dive with one of us is the FOCAL LENGTH OF THE LENS..!
If it's a wide angle, then buddy up, you will get a bunch of cool photos of yourself diving. Everyone loves those, right? Gotta have divers in the shot, otherwise you can't see how big the whale shark was...
If it's a macro lens, just clip him or her to the anchor at the beginning of the dive, go do your sprint around the reef, and then unclip your buddy when you get back and it's time to go home. We probably won't even notice that you were gone...
Mike