Water runes are one of four basic elemental runes used in many spells like water combat spells, stun, weaken and teleportation.
Water runes are the product of using rune essence on the Water altar via Runecrafting, located near southwest Lumbridge swamp.
They can also be obtained from pickpocketing Wizards, killing magic wielding NPC's, bought from a rune store or stolen from a rune stall.