how come everyone can hold their breath so long underwater in this game??!!

Because of Tidus's hair. It's all because of his hair.
 
One has to learn to stop asking questions about the Final Fantasy universe. How come a giant dark sphere is able to completely obliterate an entire world? How come a chick is able to go to temples and learn to summon ancient mystical creatures? It's not like anything else about the game makes sense...

I would definitely argue that the blitzball takes the cake for unexplained phenomena in FFX.  They never even try.

Ask yourself this: Wakka has the strength to destroy many flying creatures in this game. Why is that when he GUNS a blitzball of Tidus' head not only once, but TWICE (Besaid at the start, the boat to Luca after learning Jecht shot) in the game that Tidus feels no side effects from it? Forget about Sin's toxin, I think we all know the real source of Tidus' brain damage at the start of the game.
 
Also, mentioning fishing in a Final Fantasy forum? This guy called, he wants his scene back:

Not everyone, lulu, yuna and someone else couldn't when captured in some later level. Only tidus, waka and rikku could. Playing alot of blitzball, waka and tidus are used to always being under water. Rikku is probally used to the environment like desert sand, water and stuff like that because she's an al bhed.

Why is the Blitzball the most powerful weapon in this games universe?