Space over the ocean. We can mine, drill and exploit all the space loot from new planets, moons and other spheres. Unlike the sea, we would not have to worry about that stuff hurting our wildlife or risk making our Earth less livable. It would be the aliens' problems at worst. Or more likely no one's problem since there is a lot of spheres without any life on them.
The seas belong to the fishes, whales and mermaids. Why go to war with the little mermaid when we can just take those unclaimed spheres above us?