water worlds
I read in Worlds Without End, by John S. Lewis that given the needed properties, mass essentially determines whether or not a rocky planet will be a water planet. As I recall 3X earth mass will always be a water planet, with the likelyhood of a water planet increasing dramatically for even modestly higher mass planets.
It is an excellent book for answer your questions, (not that the answers are guaranteed to be correct).
Regarding the Earth/Moon event, it is my understanding that the impactor merged with Earth to coalesce into Earth and our Moon.