"Unleash the damp contents of your mouth on your foes!"
It is one of the four elemental "breath" abilities that cost four mana - the others being Boulder Breath, Hurricane Breath, and Fire Breath - and it is the strongest Water-element attack. This makes it a very effective weapon against monsters with the Fire element.
The enemies that can use the Aqua Breath attack include the Dragon and the Darkdragon. There are two boss monsters that can use it - the corrupted form of Hydrex (from which the player obtains the tome), and the Final Boss.