The canteen is the only portable source of drinking water the character has.  Due to dehydration from the plague, Robert must drink very frequently, (for some reason Cartography seems to increase thirst) and the canteen is the most easily accessible source of water.  

It carries enough water for five drinks.  One drink will instantly cure dehydration.

The canteen will be fully filled whenever water is drunk - from a water jug , or any non-saline water source (ponds, lakes and rivers).

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