If you actually are diving, strobe lit shots will be a lot more useful than natural light shots. If you are shooting for the classic Reef with Sun Ball/Rays, then a good (powerful) strobe with a fast sync speed is highly recommended. In fact, when shooting vertical, the 7D will actually sync at 1/320th due to the lower effect of gravity on the shutter blades. Most of the time, you'd be shooting at the lower ISO (100-400) ranges, rarely ISO 800 and up.
Listen to what Otara and Steve tell you, both are extremely experienced underwater photographers and respected contributors on Wetpixel. I used to shoot both the 5dmk2 and the 7D underwater. I eventually sold the 5dmk2 mainly for the phenomenal Tokina 10-17mm Fisheye zoom. There is no Full Frame lens that comes close to what this lens does. The Canon 8-15 only goes from a circular fisheye to diagonal fisheye while the Tokina goes from a diagonal fisheye to a 17mm rectilinear. My other favorite UW lens is the 100 EF Macro. You can see some images on my site: www.sls-photo.com
(this site is also my site for selling underwater camera gear...I'm an Aquatica dealer).
Low light focusing isn't really a big deal as well, most experienced shooters mount a focus light on their system. As an experienced diver, you know that there are lots of cool critters in the nooks and crannies of the reef, having a light on any dive is handy to spot them, so just transfer it to your housing.
As of today, no one has announced that they are supporting the D600 or 6D. Ikelite likely will but since they're a competing company to the one I represent, I won't say anything about their housings. But, regardless, you will want a good housing with proven ergonomics, as mentioned earlier, you don't want to have to go fumbling for with knob controls f-stop when the whale shark is heading your way.
Aquatica currently makes the 5dmk3 and is only beginning to ship the D800 housings next week. Figure on at least 5-6 months from camera release to a housing being released.
Best of luck, this is a slippery slope that you're going down on should you proceed.
Stewart L. Sy
Your Source for Quality UW Photo Equipment in Western Canadawww.stewartsy.com
Aquatica/TLC , Sea & Sea, ULCS, Magic Filters
When Your Image Matters.....