This is an interesting way to think about it. For one thing, the ocean is most teeming with life near the surface and almost all life in the air is on the ground most of the time.

But consider an alternative. What if the surface was inhospitable to life and likewise the shallow regions of the ocean were likewise.

Imagine something like Venus with huge mountains, like Mons Olympus type mountains. The upper atmosphere and those mountain peaks might be amenable to life.

Now imagine that same place had oceans of an alien variety. Maybe water trapped beneath the surface like lakes under Antarctica. And imagine those places could support life.

So you'd have two or more ecosystems that either never allowed life to move between them or they once were able to but no longer could.

You'd then have two environments where life could evolve completely independently. What a fascinating place that might be.