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Water Blast is the strongest free-to-play magic water spell and the third strongest water spell for members in the standard spellbook. This spell requires three water runes and three air runes to cast. Without boosts, the maximum damage of this spell is 451 + the Magic level of the caster.
It is not possible to cut the cost of water blast (and its variables) to zero, since staff of air (and equivalent staff) must be held in both hands nowadays. Even before release of Evolution of combat, while both items can be equipped at the same time, Water Blast requires an additional death rune.