If you are hauling that much water, you probably are drinking constantly and not refiling any water, so get a 3L hydration bladder with hose. The few extra oz for the hose is worth it, and hopefully your pack has a pocket to keep that 3L close to your back---the optimal location.
Now all your other water does not need to be accessible. The best place is close to your spine, higher is better than lower.
If I'm taking 10L, I have 3L in my hydration bladder, a 1L soda bottle, and a 6L MSR Dromlite bladder. The 3L bladder is in the pocket by my back, the 6L goes lengthwise on top main compartment, so that it sits approx at the height of my shoulder blades, close to my back. The 1L bottle goes wherever.