Water Bag (also called Waterskin or Bota bag) is leather storage for water or alcohol.
There are amy ways to make one yourself but I like design of this one.
LiquidI do not consider drinking from it while in fursuit or wearing it while out of fursuit.
It allows me to leave it empty but it is not the right thing to do. Filling it with water may be sufficient but it is too ordinary (ignoring the fact that having only one water bag, it should be best to have water in it).
Another option is to fill it with alcohol but I do not drink alcohol (and there is possibility of breaking the water bag and splitting it all around).
I decided to fill it with water colored by caramel or tea.