You need to break down what apparatus or skill causes each. Why one and not the other, like I have gills or don't need air -or my body switches to a secondary system when I'm in a vacuum. ??????
Almost makes no difference, but I'd take the underwater option.
Anywhere cool to swim unassisted, has sharks and more and anywhere down deep - has pressure.
So I'd basically be a paranoid snorkel-hero.
Splashing around the resort beach.
Maybe with enough weapons, I'd explore sunken ships for treasure or explore underwater caves.