The Tokina 10-17 is fine for sharks, barracuda, grouper etc, but you have to get close. Yes it forces you to get close, which is good. Remember as well, the 17mm end is in the middle of the 12-24 range. When you start to get toward the 24mm end of the 12-24, your subject will frequently be too far for great image quality. The 10-17 also focuses as close as you can get, literally on the dome. The 10-17 is harder initially to use than the 12-24, but the discipline it enforces with getting close and good lighting arrangement can result in some killer images.
I would add that the 10-17 does not suffer from edge sharpness issues behind a dome that I have found with the 12-24.
My overall experience with this lens is the same as Tim's and I have sold my 10.5. The only difference is I changed the 20mm extension for the 15mm with the Subal dome and that eliminated the little bit of vignetting I was getting with the 20mm extension with the lens at it's widest.