Now, I've played FFX many times, I've played FFX-2 several times (getting 100% completion is a pain), & I own a copy of the Eternal Calm video. Now, as far as I can tell, Blitzball players go through training so that they can perform maneuvers while remaining underwater for long periods. Tidus, Wakka, & (for some reason) Rikku can remain underwater essentially indefinitely because they have been taught how to do so. Tidus was training Yuna on how to do this during the first game, apparently, because we see her practicing her breath-holding during the Eternal Calm video. She can only do 2 minutes & 41 seconds, but it's a start.

Now, Blitzball players seem to be able to do this in Blitzball spheres & in the ocean. Exactly how one learns to hold one's breath indefinitely is beyond me. I made it a rule here that any creature gains the [Water] subtype while within the sphere. As for in the ocean, I'd say it'd be too powerful to allow Blitzers to breathe water, so I would just leave it as an issue of race. If your race can breathe water (like the Hypello), then great; if they can't, then tough luck under the sea. Maybe I'll make a Water Breathing feat of some sort, with some appropriate prerequisites. I know it's a minor departure from how the game presents the world, but it makes a lot more sense & better game balance, IMHO.