Hi Bruce-
I just purchased the new Sigma 10-22 for my d70/d2x.. It is still dry but looks promising for U/W, especially shark shooting.. May be a little less $$ than Nikon 12-24 and about the same price as Tokina 12-24..
I believe it's a 10-20mm, f4-5.6 lens. It is half the price of the Nikon 12-24, but not as sharp at the edges. Probably a good candidate for the shoot. I have this lens, and like it a lot. But you do get what you pay for -- Nikon lenses are usually sharper and more likely to be compatible with future products. They should be, at the price.
Latest gotcha: for the D200 many Sigma DX lenses have to be re-chipped to work with one new function on the camera, the rear AF switch, including this one. To their credit, Sigma is offering to do this no charge, but you lose the use of the lens for a period of time. Again, this is only for the new function on the D200, and does not matter for the previous Nikon DSLRs.