The Astral Sea
The Astral Sea isn’t a sea at all. It is a silvery nebulous body that stretches out connecting various islands and realms. It has a natural sort of horizon which creatures travelling through tend to orient themselves to even without realizing it. Darker silvery mists make the ‘sea’ while thinner more open mists make the ‘sky’. Creatures that breathe can do so in the plane and those that swim or fly do so respectively as there is not really a substance to the silver mists and it does not impede travelers.
The Astral Sea holds the realms of the gods and many various ‘islands’ float in the void. Only some of these realms are in a fixed location such as Hell or Celestia. Others float freely or orbit more stable realms.