[njarboe]

Earth likely had some water during early creation (accretion) but the best theory on how the Moon formed is that a Mars sized object hit the early Earth at the end of accretion. This blasted much of the Earth's rock part (mantle) and any water it might have had into orbit. The rock part quickly fell back to Earth and the rest condensed and collapsed to form the moon. The water and other volatilizes that were on Earth were blown away by the solar wind at that time. The surface of the Earth and Moon would have been boiling lava at this point and had to accumulate water from new impacts of comets and icy asteroids.

The water on Earth may seem to be a lot to us on the surface but it is only about 0.02% by mass [1].

https://www.universetoday.com/65588/what-percent-of-earth-is...

[kobeya]

There's actually a lot of water in the mantle of the Earth. There's something like 3x the current ocean volume trapped in the mantle, and there are theories that pressure differentials cause this water to escape as gaseous water escapes from the atmosphere. It would explain why the ocean happens to be approximately the height of the continental shelf, and always has been. Under this theory the water simply emerged as the Earth cooled from the impact. The Moon, on the other hand, would be missing this water as the debris was mostly stripped of water content as it accreted.