Just as when you’re pulling nickel out of the ground in Sudbury, when you use ocean water you’re mining asteroids. As I noted in my latest essay, the more we learn about the solar system, the more we discover that, as opposed to being what we long thought was “the water planet,” earth is a comparative desert. The water is mostly extraterrestrial.
To expand on Krafft Ehricke’s famous statement, if God had wanted us to become space faring, he’d have given us a moon. With water on it.