Rikku can hold her breath for around 10-15 minutes underwater. This is somehow a skill that certain people in her world learn, and not a magical ability, but I have no fucking idea how it works. Just that out of your seven playable characters, only three of them can do it. (It's not just the three of them: there's an entire sport in her world that takes place underwater.)

They don't really explain it, just - some people can hold their breath for 15 minutes, and those are the ones you use when exploring caves.

She's also got a knack for mechanical things, but that's really anticlimactic after the "apparently doesn't require oxygen and/or has gills" thing.