LChan,
I'm pretty sure this has been discussed at length before, so you might want to do a search.
My view: I have the 17-35 mm and use it a lot, underwater and above. This is a magical lens. It's not just sharp - colors seem (to me) cleaner and slightly more saturated through this lens, and it seem easier, main subjects 'pop' and landscapes in the right light glow in ways that I really like.
Underwater , I particularly like to use it for sharks and other animals that I'm not confident that I can approach closely enough with the 10.5 or 12-24 lenses. This is the factor that most people cite, of course. Cranked in to 35, it's roughly the equivalent of a 50 mm lens on a full-frame SLR and can do quite nice fish portraits, etc.
Another big advantage of the 17-35 is that this is a 2.8 lens. The one stop difference (compared to the f/4 12-24) can sometimes be very important in low/dim lighting wide-angle situations where it's difficult to get a good ambient exposure at f/4, but which aren't suitable for the 10.5 FE (also f/2.8).
The length of the 17-35 is almost identical to the 12-24, so you should be able to use it with a big dome and the same extension ring (and diopter, if you use one) as the 12-24 mm zoom. I can't remember if the lens takes a different zoom control ring, and that may depend on the housing. (On my Subal D2X housing, the zoom control works slightly better with the 17-35 than the 12-24, which sometimes can slip at depths above 15 meters.
So. I love this lens, am very glad I have it and use it a lot. Whether I would buy a new full-price 17-35 now, however, is another question. I purchased the 17-35 back in film days, to use as my main wide-angle lens with the F100. This is not a cheap piece of glass. But it's a good one.
Frogfish (Robert Delfs)