What lens to buy?

I have a Nikon D200 and want to know what mm lens to get that would take a similiar picture to what I use to take with my Nikonos 5 with a 15 mm lens. I will be using the Ikelite housing.

i would suggest the Nikon 16mm or the Sigma 15mm

Or perhaps the new Tokina 10-17 zoom might be to your liking if you want more versatility from one lens. This one is fisheye at the widest and more like the Nikonos 20mm at the 17 end

Nikon 12-24. My wide angle lens of choice.

You might add the 10.5. It's small and not too expensive a lens. If you don't mind that it is a fish eye.

I no longer use my 12-24 underwater; I cannot get the sharpness I like.

I love my 10.5, my 16mm and 17-55.

But.....the Tokina 10-17 appears to be the lens to beat for a versatile underwater wide angle lens.