Your reservoir of magic allows you to breathe normally even underwater.
Prerequisite: Ability to cast 3rd-level spells.
Benefit: As long as you have a water spell of 3rd level or higher available to cast, you can breathe normally in both air and water. This supernatural quality requires no activation.
As a secondary benefit, you gain a +1 competence bonus to your caster level when casting water spells.
Source: Complete Mage, page 39.