Like the title says, it's a Leather Water Sac that can be made from TFC items.
It has 12 uses (which is about 2 thirst bars worth) before you have to fill it up again.
To clarify my view on modpacks, if you are using TFC, you are more then welcome to use the Leather Water Sac. Have fun
All credit goes to original creator of the mod Emris