To last longer underwater.
Air Potion +
The Air Potion is a potion available in Skyward Sword. Link can buy it at the Bazaar from Luv. It costs 20 Rupees and slows the depletion of Link's Oxygen Gauge for three minutes.
The Air Potion + is also available as an upgrade if Link takes the Air Potion to Bertie along with rupees and certain kinds of bugs.