Take a look at the camelbak MULE if you haven't already. I use the milspec versions for my wife and my GHB's and they hold an 100oz reservoir plus ~540cu in of cargo space. External dimensions are around 17 in x 9 in x 7 in. If you are lucky, you will find a milspec version - mine both are - that use 1000D instead of 500D nylon and are a bit more rugged. They are hard to find these days, though, the civilian 500D is much more popular. Here's an arfcom thread from a few months ago where someone still found a 1000D version:
http://3gn.ar15.com/archive/topic.html?b=6&f=10&t=364891
The 500d does have a waterproof, wool lined pouch for camera/nvg, but i prefer the '1 huge ass pouch' approach of the milspec bag - it gives you a big more usable space - overall cargo capacity on the mil version without 2 compartments in 1000D is 540c in, whereas the new civ ones are only 488cu in.. My friend ordered one, and got a 500d version so i've seen them both in person. The easiest way to tell the difference is by looking at the bottom straps - the milspec 1000d version has 'wings' that let you fold the waist strap up into little pouches whereas the civilian versions don't.

They have MOLLE webbing on the front that I have used to mount a detatchable maxpedition pack w/ essential medical gear in it. If you don't carry much in it, you might not need the extra space at all.

civilian version of camelbak pack: http://www.camelbak.com/Military-Tactical/Packs/2011-Mule.aspx

milspec looking pics - notice, 1 top pouch and 'wings' on the bottom strap attachment point for folding up the waist straps:
http://www.nalpak.com/Camelbak-MULE-MG?utm_medium=shoppingengine&utm_source=amazonproductads
Also, the civilian version has 2 rows of velcro webbing right against eachother on it's MOLLE area...whereas the milspec has molle-velcro-molle-velcro-molle...

addon pouch: http://www.maxpedition.com/store/pc/5x7x4-Horizontal-GP-Pouch-7p761.htm