I have both these lenses, and use both of them. I've done shark and manta dives where the big guys either don't show up or don't let me get close enough for the 10-17. With the 17-70 I can zoom in some, or go off and shoot other things without being stuck really wide. Yes, the Tokina is better for truly wide angle, but if you're not sure what you'll see on a dive, the versatility of the Sigma is helpful. But as long as I have the 17-70 behind a dome, even at 70mm I still don't really get close up "macro". But it's good for fish portraits and small reef scenes.
I carry a Panasonic Lumix as a backup camera. Without a housing it is rated to 48 feet, but I've had it down to 100 feet. It's useful having a waterproof point-n-shoot on the boat deck. I've used it on dives where there were both macro and wide angle subjects expected: my SLR system setup for macro, and the Lumix in my BCD pocket for the occasional wide angle shot. It can't get me closeups of moving subjects because of shutter lag, and the onboard flash produces backscatter sometimes, but it is surprisingly capable. When I flooded my SLR on day 11 of 14, I was able to take photos the last three days with the point-n-shoot.