Tamron 17-50mm Lens?

I am thinking of getting the Tamron 17-50mm as a general purpose scout lens. Does anyone have any experience using this lens? Has it worked well underwater?

I am not sure what configuration to use with this lens (for a Canon Xti & Ikelite case setup). Ikelite's website lists two ports – the standard dome (5503.55) and the modular dome (5510.28 with 5510.45). I am leaning towards the standard port, but I am not sure if I need to use a diopter with it. Ikelite's port chart says it will focus close enough that a diopter isn't required, but a +2 diopter may enhance corner sharpness. Should I be using a diopter? Will the standard port still be the correct size if I add a +2 diopter? I have never used a diopter before, any suggestions?

Thanks for your help!

I bought this last month for my 40D and used the 8 inch dome.... I was not happy with the dioptor.

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I really wanted to get this too as a general purpose lens for my D300. I used it with my old Sony Alpha and loved it. However, because of dive use considerations, I bought the Sigma 18-50 EX F2.8 HSM Macro instead. The macro ability means I don't require any diopters. The lens seems ok but I think the Tamron may have been a bit sharper.... I could be wrong