I was shooting my 7D in the Nauticam housing using the Tokina 10-17mm rolled to 10mm 95% of the time. There is so much stuff in the water that I felt like I wanted to limit the water column as much as possible. Especially if you follow Adam around. (See above) Others were more comfortable at 17mm when shooting the crop frame. Guess it depends on the look you're after and how comfortable you are getting close to the sharks without touching them. I used my little Zen dome for most of the trip (easier to free dive with) and put my big dome on for two days of shooting splits. I was shooting manual exposure, hi speed continuous the vast majority of the time.
The sun angle you shoot is probably more important than the lens. Shooting across the sun provided me the best opportunity to limit backscatter. I had the best luck down or across the sun early in the morning as you'd expect.
I spent about 5 hours in the water every day in my dive skin and was very comfortable. Take
big huge cards with you. One unforgetable morning I shot 500 images in less than two hours while being constantly circled by whale sharks and dancing mantas. That will just about fill a 16GB card on the 7D.
My Isla Mujeres set on Flickr is still in progress but a few images are up here.