Christian significance of Tolkien and C. S. Lewis putting "heaven" across the sea?

It is interesting that both of the most prominent Christian fantasy authors (not as in "Christian Fantasy" but Christians who wrote fantasy) put paradise across the sea. I realize that Tolkien's Númenor (the place across the sea in Middle Earth where Elves go, etc.) was not actually an analog of heaven (despite my title's bait wording :), and probably traces more of its mental origins to Celtic myth and legends of Atlantis. And I imagine that C. S. Lewis may have derived his idea of Aslan's country from discussions of Númenor with Tolkien as part of their Inkling 's group. *But* I'm wondering if there is anything in earlier Christian writings or thought (especially the Bible of course) that might have influenced this creative choice? I also find it interesting that "across the sea" seems to have been picked up by some Christian musicians (Jeremy Enigk's "The Prophet " and I think there was one by Havalina Railroad Co which I can't find, there may be others). So, lest this be considered off topic, I repeat: what is the *Christian* significance of the idea of a paradise across the sea (if any)?