The Nikon 17-55DX is a superb lens above and below the water. With a diopter, you can shoot anything from shrimp to whales in crisp focus. Most of the sharks I have seen (Guadalupe, Cocos) would not get close enough for a 10-17mm, or 12-24. With new sensors providing greater exposure flexibility, I would expect that the lens will become even more useful for subjects beyond the range of a strobe.
This lens is DX only so there are good deals available on them at the moment as many Nikon pro shooters are offloading them. Personally I would not consider this lens as particularly wide, yes you can shoot subjects up to about the size of your buddy, but you will really be pushing it. I imagine that you could take a photo of a whale, but it is unlikely to be a great one. As for shrimps it would have to be a big shrimp.
I have done a few dives with this lens, but ended up buying the Sigma 17-70mm (which I have done more UK dives with this year than any other lens). The Nikon is a better lens, but the Sigma has better close focus and offers amazing performance for the price. It is also a DX lens.
Alex