Water flask

The water flask is a relic of the vaults. In Fallout the Vault Dweller is given three flasks if the Outdoorsman skill is tagged. At the beginning of Fallout 2 the Chosen One is given a flask from Vault 13, to assist in their search for the G.E.C.K. It is worth 25 caps in the first game.

Fallout
The flask prevents dehydrating random encounters from occurring. A large number of flasks (46 in the footlockers) can be found on the third floor of Vault 13 in the Vault's water supply, but few outside of the Vault. The water guard carries 3 extra water flasks.

Fallout 2
In the second game, the Chosen One can ask Vic where he got the flask, which leads to them being directed to Vault City to see Ed the brahmin dealer for further information. Showing Ed the Vault 13 flask will put the Den, Modoc, Broken Hills, Redding, and New Reno on the world map, and grant 500 XP.

Fallout: Brotherhood of Steel
A blue flask for holding water, with a number 13 inscribed on it. The flask can only be given to the wasteland stranger in exchange for 100 bottle caps.

Fallout: New Vegas
A standalone water flask can only be obtained with the Classic Pack add-on. However, a canteen is present as a part of specific items of armor and clothing in the standard Fallout: New Vegas game.

The canteen's primary function is to help stave off dehydration in hardcore mode but it also regenerates a small amount of health in both normal and hardcore. However, it cannot be used as the only source of water to stay hydrated.