I'll definitely choose the ability to fly. Sea/ocean is an entirely alien realm upon entering which you lose track of the land and it is almost like venturing into the outer space. And then there are various unknown creatures lurking there, scary as hell. It's also guaranteed to be totally dark after just a few hundred metres from the surface. On the other hand sky will get dark only after sunset, there are no scary creatures flying there, you can always see the land at any time during your flight and it's so reassuring lol. So yeah, ability to fly for me.