I recently added a molle compatible Hydro Carrier to it.
The Ruck has already a H2O Slot.
I use the additional Hydration Carrier as extra room. The Carrier has also molle webbing. crazy.
http://www.ebay.com/itm/Condor-HCB-MOLLE-Water-Hydration-Carrier-w-2-5-liter-Bladder-Detachable-Straps-/201188061164