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.