The spell requires a reagent that is dropped by certain mobs: Shiny Fish Scales. Glyph of Water Breathing will remove the reagent cost.
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.
Tips and tactics
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.