Water flask

The water flask is a relic of the Vaults. In Fallout the player 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 GECK. It is worth $25.

Fallout
The flask has the simple property of stopping "dehydration" random encounters from ocurring. A large number of flasks (50 or so) can be found on the third floor of Vault 13 in the Vault's armory, but few outside of the Vault.

Fallout 2
In Fallout 2, the player can ask Vic where he got the flask, which leads to them heading off 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 give the player 500 XP.

Water flasks are a much rarer item in Fallout 2; with one being given to the Chosen One before they leave on their quest and a small number being found in Vic shack, but outside of the main quest they serve little purpose.

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 can not be used as the only source of water to stay hydrated.