User blog comment:IamJakuhoRaikoben/Chapter 291: Naval Battle, Review./@comment-82.145.210.149-20120712221633/@comment-4866818-20120712222426

There's oxygen in the water; it's possible they altered the amount of oxygen, this allowing more air for the Mages to breathe in, or maybe they did something with the pressure in the water. Probably the first one, maybe.