It would be nice if we knew your rig.
The Sigma 17/70 HSM has a minimum focus distance of about 8" and a magnification at that point of 2.3:1 at 70mm. The lens extends out quite a bit at 70 mm and the working distance is about 2". At distance I can get 1:1 magnification with a 60mm. Also the largest f/stop is F/22 and I often shoot well above that for serious macro. The botttom line is that this is a semi-macro lens. On the other end, my wife uses an Olympus 14/54 which has approximately the same specs once you convert the 1/2 size sensor of the Olympus to the DX size and she takes pretty good macro shots. Understand, however, that every time we take the same macro shot my Nikon Af-105-VR or 60mm shoots a better picture so dont expect miracles!
On the flip side, my wife also takes pretty good semi-wide angle at 17 mm. She can get a decent picture of a nice reefscape or a significent sized critter while I am just shooting the eye. Still, if I mount my 12/24 and dome I can get a wider, closer shot so don't expect miracles here either.
The benefit is that you can do both with a 17/70 in a flat port.
HOWEVER, the new sigma 17/70 is a VERY fat lens and mnay housing/port/zoom ring configurations won't take it. In fact Ryan of Reef Photo is currently working with my 17/70 and SeaTool D-300 housing to see if he can work out a marriage of the two by permanantly affixing the zoom ring to the lens.
Hope this helps