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:
Manta 3/4 Overhead by Optical Ocean, on Flickr
Manta Head shot by Optical Ocean, on Flickr
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):
Manta Eye level by Optical Ocean, on Flickr
Friendly Dolphin by Optical Ocean, on Flickr
Hope that helps!
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