Waterskin

A Waterskin carries up to four servings of water, which is required in the Kharidian Desert. Without a waterskin, players will lose health in the desert due to the desert heat effect.

Full waterskins are dropped by Kalphite and can be bought from various Kharidian NPCs.

Empty waterskins are dropped by Desert Lizards.

Players can refill them by using them with water filled jugs, buckets, or bowls; or using a knife with certain cacti. Cutting a cactus in order to fill a waterskin will yield 10 Woodcutting experience points.

Also the Lunar spell Humidify will refill all waterskins in a player's inventory, making it possible to stay in the desert indefinitely with waterskins.

Since many quests are set in the desert, this is a required item for many quests.