There really isn't a good all around lens for underwater work in the dSLR realm. The issue with the 17-55mm is that, as Mark points out, it is not really all that wide, especially on a cropped sensor. With that focal range, you're likely going to be doing portraiture so you may as well have a 60mm macro on and have the added flexibility to shoot macro. If you're thinking of shooting largish animals like sharks and mantas, I prefer a nice wide angle or fisheye, and if you want the flexibility of a wide angle zoom, the Tokina 10-17 sounds like a good choice.
Just remember that it is hard to have one lens to do a good job on a lot of different things. The nature of shooting dSLR is that you have to really pre-determine the kind of dive and shooting you wish to do, especially if you really want to get the awesome shots.
One thing that might be an advantage with a f/2.8 lens is to aid autofocuing, but other than that, shooting wide out at f/2.8 isn't really going to be much of an issue unless shooting ambient light.
Just remember that it is hard to have one lens to do a good job on a lot of different things. The nature of shooting dSLR is that you have to really pre-determine the kind of dive and shooting you wish to do, especially if you really want to get the awesome shots.
One thing that might be an advantage with a f/2.8 lens is to aid autofocuing, but other than that, shooting wide out at f/2.8 isn't really going to be much of an issue unless shooting ambient light.