Water skins are lightweight pouches used by the people of the Avatar World for the storage of water. For waterbenders, they also serve as sources of bending material in situations where water may be scarce or unavailable. They can vary greatly in size and model.
A water skin is advantageous to waterbenders in combat, especially in duels or large-scale battles, as it provides a portable source of water that can be easily accessed. When several are used in concurrence, they can provide tremendous amounts of water, making up for any lack of the substance in a given environment. Although water skins present the dilemma of retrieving more water whenever the supply is depleted, they can be easily replenished, and exhausting their contents can be avoided by bending used water back into the skin.
As a prominent user of these pouches, Katara demonstrated on many occasions the importance of a water skin. For instance, while in the Fire Nation occupied stronghold of Omashu, Katara was able to effectively defend herself against Mai and Ty Lee until the latter blocked her chi, due to the supply of water readily available from her pouch.
Also, during Team Avatar's attempt to break into the Earth Kingdom Royal Palace, Katara used water from her water skin to create an ice slope that elevated her, allowing the creation of a much grander water whip with the water from the moat. With only the water from her pouch and the amount available within the vicinity of the palace, Katara was able to defeat several units of Royal Earthbender Guards.
During the invasion of the Fire Nation, Katara used three larger water pouches to compensate for the lack of water in the Fire Nation Capital. With these pouches, she was able to snap the chains of a ballista and defend herself against the attacks of Imperial Firebenders.
The different sizes of the water skins also allow people to carry them underneath their clothing and thereby launch a variety of imperceptible attacks. When Avatar Korra faced three Triple Threat Triad members, Viper was able to assault her instantaneously with a water bullet from a hidden pouch.
Chief Unalaq also carried a water skin, which he used to defend himself against his niece and Mako during an altercation at the Southern Water Tribe prison, and Kya used a water skin to defend herself against dark spirits during the Siege of the South Pole.
Ming-Hua carried several water skins under her robes when the Red Lotus infiltrated Ba Sing Se in search of the Avatar, using the supply of water to incapacitate a Dai Li agent after they confronted Earth Queen Hou-Ting.
Chief Tonraq carried two water skins with him to the Northern Air Temple. Fastened across his back, the waterbender used the contents of the pouches to create ice hooks with which to scale Laghima's Peak and he later fashioned the water into tendrils in order to battle Zaheer alongside Korra.
In cases where healing is necessary, it is advantageous to have a water skin, as a body of water will not always be present when someone is injured. Water skins allow waterbenders to treat wounds sustained during a battle as soon as they appear, making the healing process more effective.
For instance, while searching for Appa under the waters of Lake Laogai, Katara was immediately able to work on a wounded Jet after he was struck by a fatal earthbending attack by Long Feng. Similarly, during the invasion of the Fire Nation on the Day of Black Sun, an injured Hakoda was effectively treated by Katara, as she had a source of water readily available from the three water pouches she carried. Having water available to her also proved to be Zuko's salvation during the Day of Black Sun, as Katara was able to bring him from the brink of unconsciousness after his unsuccessful attempt to redirect a lightning attack generated by Azula.
The presence of water skins during long periods of travel can be beneficial, as they provide a source of water from which to drink. Wan carried a water skin when he left the aye-aye spirit's oasis on Mula to explore the Spirit Wilds, replenishing the pouch periodically when a body of water was nearby. A Red Lotus sentry also carried a water skin while guarding captive airbenders near the Northern Air Temple.
Water skins are especially advantageous to have in arid conditions where there is a limited supply of water. For example, Zuko used a water skin while traveling through arid terrain in the Earth Kingdom. Similarly, deep in the harsh Si Wong Desert, Team Avatar was able to survive, albeit narrowly, with the water available in Katara's water skin.
Water skins can also be useful in missions of sabotage, as evidenced when Team Avatar infiltrated the Fire Nation drill headed for Ba Sing Se in an attempt to dismantle the machine by targeting its interior frame. Consisting almost entirely of metal, the water skin provided Aang and Katara with enough water to slice several of the metal beams within the mechanism, in order to weaken its frame.
Later on, Aang used water from the skin to carve a target onto the drill's center-most point, where he later used a combination of airbending and earthbending to accumulate enough momentum to render the drill inoperable.