Quote Originally Posted by feona17
I read somewhere on this forum, that the water in Spira is different. That's why.
The standing theory around the FFX populace is that the pyreflies have properties that alter or oxygenate water, so that players can play underwater. It's not really a question of breathing, it's about being able to hold your breath for a long period of time.

While I myself have accepted this, the idea seems a bit sketchy, and leaves many questions unanswered.