If all you have is the 10.5mm and 60mm I would highly recommend getting a midrange zoom. If you do much topside shooting you are going to need it.
How much topside shooting do you do? If you are like me (and most people) the majority of my shooting is topside (although my purchase decisions are skewed toward UW). If this is the case I would highly recommend the 18-200 mm VR. This is not a lens that you will use UW, but it is the lens you will use most frequently. This lens is very good despite its wide zoom range. See any of the reviews online for another opinion.
If you must have the absolute best and money is no object and you don't mind carying lots of glass I'd get the 17-55 and the 70-200 VR. both for topside. I know there are some here that use the 17-55mm for shark photography.
If you are really concerned only about underwater, or you plan to get a midrange zoom for topside anyway, then the 12-24mm really is the way to go. Nitpicking about corners aside this is a very good lens, fast AF-S and just the right focal length for WA shots underwater. And to put this in perspective, the corners on many of the old mainstay wide angle zoom lenses on full frame (or film) are not any better. We're just all spoiled by their performance on cropped sensors. BTW, the Tokina 12-24mm has very good reviews at 1/2 the price and desreves a hard look as an alternative to cover this range.