I don't know anyone who has used the 17-70 for that (not saying that it can't be done, of course!)... the few examples of macro underwater shots that I found online weren't that impressive, but maybe that's because not as many people use it.
It is certainly a reasonable, economical topside lens if you want to be able to shoot a wide range of subjects. It is nowhere near as fast as the Tamron (f2 vs f4 for the Sigma at the longer focal length), and I don't think that you will get much background separation ("bokeh") with a slow lens like that.
Also, you would use a dome port for this instead of a flat plate, which means that you would lose the magnification factor that macro photographers look for as well. I'm not an expert on this, but I know that there is a difference..
If you really want to do macro, get the prime Tamron 60. If you want to take photos of a wide range of subjects and have the option of getting close on occasion, it might be a good compromise. Maybe someone here has used it...?
Mike