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Water Bolt is the second strongest free-to-play magic water spell and the fourth strongest water spell for members in the standard spellbook. This spell requires two water runes and two air runes to cast. The base damage of this spell is 220, and it scales up to 432 at 45 Magic.
It is not possible to cut the cost of water spells to zero, since the staff of air (and equivalent staves) must be held in both hands, and cannot be wielded together with the tome of frost. Even before the release of the Evolution of Combat, while both items could be equipped at the same time, water spells required additional mind, chaos, death or blood runes.