Vault 13 Canteen

 is a miscellaneous item in Fallout: New Vegas.

Characteristics
The canteen essentially stores an infinite source of water, automatically lowering the player's H2O meter on a regular basis, and is useful primarily to ward off dehydration in hardcore mode. The canteen can also help with the Achievement/Trophy "Hardcore".

In both hardcore and casual mode, as long as the canteen is in the player's inventory, the player will periodically receive a message at the top left corner, "You take a sip from your trusty Vault 13 canteen", and the H2O meter will drop. Since H2O is not important in casual, non-hardcore mode, this may become more annoying than useful.

The canteen seems to trigger the player's drink and subsequent notification when the H2O meter reaches a certain level (something just under 200 where dehydration would set in). However, under certain circumstances - such as sleeping or fast travelling right before the hydration level reaches the trigger point - the player can "jump past" the trigger point and become dehydrated. The canteen will not work again until the player lowers their hydration level below the trigger point using some other means.

There is no way to deliberately force a drink from the canteen. It only works as an automatic hydration source.

There is no player animation associated with drinking from the canteen; the screen notice and H2O meter drop are the only indications it is working.