You can use the 10-17 + the TC and get a little more reach. I've found this to be a sharper solution than the Sigma 17-70 which is about the only other lens I like underwater. The 17-70 is fairly useful and has a good range including sort of a close-up mode. But I don't think it's got stellar sharpness.
I've shot all three of these solutions down there. Mid-range zooms tend to be slower lenses and don't shoot very well uw (poor corners, etc.). Socorros can be murky at times.
I think a larger dome does give other options, and you can shoot at more wide open apertures than in the mini-dome, so I would recommend looking into that. The 17-70 requires a large dome and a 50mm extension, so it tends to be a bit bulky for shooting in blue water. Shooting a 60 behind a dome is a decent sort of shark setup for more reach, fairly sharp and fast, no extension.
Here's a couple of examples shot there.
Sigma 17-70 D300:
This was at 29mm, and could of been shot wider, but the Manta was moving quickly.
Tokina 10-17, 7" Dome, with D300 (same trip):
Hope that helps!
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