(but seriously, one detail I loved about FFX is that only some characters could participate in underwater battles, a nice integration of narrative and mechanics making it clear that underwater brawling is not an universal skill)
Oh, yeah, no, it was definitely a matter of narrative integration. It certainly wasn't because three animators died attempting to merely conceptualize how Lulu's hair/weapon/beltpocalypse would work in a floating/swimming environment. That was just a coincidence.
(But seriously, Final Fantasy 10-1.5 confirms that "super swim" is an ability that can be learned, and Yuna is attempting to pick it up during her time off from heroing. Presumably Lulu and Auron were never into it.)