[Pitchwife]

It depends, I think, on which stage in the development of the Legendarium we're considering. In the published Silmarillion Ekkaia is obviously an ocean, the Outer Sea that surrounded all of Arda, i.e. both Aman and Middle-Earth (contrasted to Belegaer, the Great Sea between Aman and Middle Earth, which was Osse's domain).

In the Myths Transformed section of Morgoth's Ring, however, Tolkien equates Arda with what we would now call the Solar System (the planet Earth, as a part of this, being called Ambar, the Habitation). So if Ekkaia at this stage is still meant to encompass all of Arda, rather than Ambar only, it can be little else than what we call Outer Space.

Which makes Ulmo's association with it somewhat confusing. Maybe somebody with a deeper knowledge of Tolkien's late cosmology (and a copy of HoME X at hand to quote from) could shed more light on this?