Does a waterskin let you carry an extra unit of water? Do players not start with water unless they buy a waterskin? My confusion continues
I agree that the writing on how consumables, the 4 special containers (knapsack, backpack, waterskin an quiver) and equipment slots work is ambiguous and/or lacking.
My take is the same as Fenhorn's: these special containers don't let a character carry more items than what they're allowed, they're just about how a character is carrying food, torches, water and arrows, respectively.
Additionally, I rule that a character can't carry water without at least one waterskin, or container similarly designed, and I emphatically enforce the rule that says a character can't carry more than d12 of a given resource, no matter how many quivers, backpacks, knapsacks or waterskins they have on them.