I'm not saying that humanity could do this today. I've been (was?) an athlete for 17 years so I know about oxygen supplies under duress. But saying "this is only possible because fiction" discounts the fact that people can do ridiculous things underwater and perhaps if it were shorter durations, or if they were able to surface, or whatever, would make Blitzball viable in our universe. It is, of course, possible in Spira because ~magic~ but that does not mean it is entirely impossible here.

Why do we question anything, Skyblade? Because we want to. Because it helps us learn. Because it interests us. Because we have things we want to know and understand. Simply saying "it's fiction" is the same as saying "God did it" and science hasn't gotten us this far by saying God did it.