Mental Quickness (Passive) Increases spell power by an amount equal to 50% of your attack power, and reduces the mana cost of all instant cast spells by 75%. Must be specialized Enhancement for this to apply.
Warlocks have a similar spell, Unending Breath, that requires no reagent. If there is a Warlock in your group and you do not have the glyph to remove the reagent cost, allow them to buff. In addition, Warlocks can also obtain a Glyph which increases swim speed if buffed with Unending Breath, another potential benefit to letting them do it.
/Patch 4.0.1 (2010-10-12): Removed from talent tree. Now trainable at level 46. Patch 3.0.2 (2008-10-14): Water Breathing is no longer a Magic buff, and thus can no longer be dispelled.