There has been much discussion about the new vs old 60mm lens here. Here is a link to some of it.
old vs new 60mm
I have the D90 as well. I have only tried the new 60 in the pool so far (changing that next week) and at times it focuses so fast it shoots past the focus point and thus 'hunts' in and out. I had to 'hit' a contrast line on the subject in the focus window at times to make it lock in. A focus light helped a lot with this issue. This testing was in an indoor pool that was fairly well lit but not bright - about the light I get in most dives.
You can use a tokina 10-17 with a 1.4x extender to approach the field of view of the 12-24 but with closer focusing. This requires an extension on the dome to account for the space of the 1.4x. As to whether this fits your needs, is as sharp as the rect. etc. I cannot say. But for someone on a budget, its one less lens to buy for now. The 10-17 can use small domes which generally cost less and pack easier. The small dome does not work with the 12-24 however so if you start with the small dome, later you might want the larger dome and end up buying 2 rather than one.
I took the DSLR plunge a yr ago and started using the 10-17 first, sometimes with the 1.4x. I love using it so much I have yet to use the 60 or 105 in the ocean even though I have the setup for both but then I do not get out that much, sadly. That should change in the next 2 weeks.
Check out the JimSwims entry in Photo Showcase on this site from a day or 2 ago for some very nice pics with the 10-17, 1.4x and small dome. He is not using an IKe housing but that should not have much impact.
Hope this is some help.
Dave