AlexB wrote an answer about Mountain Hardwear South Col 70 OutDry Backpack - 3975-4275cu in on April 8, 2014
A reservoir/bladder may be carried in the floating top pocket or in the front zippered pocket (though the latter is not ideal for weight distribution). Water bottles may also be carried in the top pocket, front zippered pocket and/or front crampon stash pocket (presuming you're not carrying crampons).
This is a mountaineering pack so Nalgene water bottles are usually carried inside the pack since they tend to freeze at higher elevations if they are outside the pack. Even though it's a mountaineering pack, the reasons you noted (weight, size and the fact that it's waterproof) make it a great backpacking pack as well!
Alex @ Mountain Hardwear