Well, yes, water is #1 of the "10 essentials".
Especially when you are sweating it out like crazy. I have thought about getting a pack bladder for the extra water, but I have worried about the bladder breaking or leaking onto the camera equipment. I like at least one bottle on a carabinier hanging off my belt - very easy to get at, and I remember to drink whether I think I need it or not.
Concerning size of pack, 60 L, plus a bear canister hanging off the pack or under a floating lid, seems about right volume-wise for long weekend trips.
At least one pro photographer, granted, a large format photographer, uses llamas as pack animals, and the recent Backpacker magazine had a funny article about goats as pack animals. (hint - goats are more stubborn than mules and donkeys).