Originally posted by I am a Sock
Anyway, what's this 'sea' your talking about, Exa?
~according to Tolkien's drawings and also his descriptions, mainly in the Ambarkanta, under the (plain, flat) earth there is a wide Sea, called Vai, Vaiya or Neni Erùmear; the earth is swimming on it, carried and held by the "words of Ilúvatar"; the oceans are just places where the earth is so thin that this water is able to reach the surface.
In other versions, this Outer Ocean isn't really water, but rather something neither liquid nor a gas like the air, and its "so thin" that only Ulmo's creatures can swim in it, and it's all around the earth in different layers above Ilmen and Vista.
I'm not exactly sure what happened to it when the world was made round after the Downfall of Númenor ~ the drawings from The Shaping Of Middle-Earth show it all around the now globe-shaped earth like in our days atmosphere, stratosphere and so on
Which kinda proves my idea nonsense 😛 that would only work if the world would still be the flat ship of old.