For underwater shots of anything larger than about a foot, you will want a wide angle lens. Your strobes no matter how big won't travel much more than 3 feet so shots of divers, turtles, grouper, sharks up to reefs and wrecks will need a WA lens. Fisheye lenses are the standard choice because the fisheye curvature really isn't as weird looking underwater as it is above. Partial fisheyes like the 15/16mm on DX or the 10-17 are ideal. For rectalinear lenses you will need a diopter behind most domes. Fisheyes won't so that's another reason underwater photographers prefer fisheyes. Unfortunately, I'm not aware of any AFS fisheyes. So you are stuck with rerctalinear lenses.
The new version of the Tokina 11-16 mm will focus with the non-focus motor bodies. I have this lens and traded it for my Nikon 12-24mm. I have been very happy with its performance. The F2.8 is great for indoor photos as well as wreck shots using filters. I believe it is sharper than either the Nikon 12-24mm or the 10-24mm. In fact, I think it is the sharpest DX wide angle lens available. None of the full-frame rectalinears will be wide enough on DX either. You need to be "unreasonably" close to your subject to get good underwater shots. :-)
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