I found the Sigma 17-70mm a bit useless underwater as its not wide enough for me for wide angle and its not that good for macro on the other end as you have to use it with a big dome. I think there are few others are like me who bought the lens as it was discribed as a great general purpose lens and ended up using it for a few times before retiring it to the collection of dust gathers.
Just to help Simon, the OP, there are a few discussions on WP if you search around about the usefulness - or otherwise - of mid-range zooms.
I'd be wary of suggesting what the consensus might be (!), but there is a school of thought (and I'm one of them) that, generally, if you are diving to take serious photographs, rather than diving with a camera to record what you might see, it makes better sense to chose the lens for the occasion rather than go for a mid-range.
I'm with Mark, those mid-ranges are great above water - but underwater they always seem either not wide enough or not macro enough. After much trial and error, I came to the conclusion that, for me, it's either shooting reefs/divers/wrecks and then the fisheyes or fisheye zooms are great; big critters/wrecks with wide-angle rectilinears - or macro.
I find u/w lens choice very different from topside. Topside it's rarely an extreme lens. But underwater the effect of water density, loss of light/colour transmission and the need for proximity to subject impacts significantly on lens choice and pushes selection to the extreme.
(I can't believe I was able to have those thoughts - and write them down - so early in the morning.... )