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)?
Before Elijah was taken away into heaven, he “crossed the Jordan,” and ever since, “crossing the Jordan” has been a metaphor for dying, or crossing over into heaven. By the time of Pilgrim’s Progress (1670s or so) this was an established metaphor for gaining entry into paradise.

Now, having seen the Jordan River firsthand, I can tell you that it isn’t much larger than a creek in many parts, but in the Western European imagination it grows wider every day. As such, it being compared to a sea (long a metaphor for the underworld of death) is not a stretch.

It should also be noted that baptism — immersion in water, even if not practised that way — is explicitly tied with death and paradise in both in the Baptist and Episcopal traditions. In Baptist understanding, when one is dunked under the water they are said to be “buried with Christ and raised again.” In the Episcopal baptism ceremony, explicit reference is made to the parting of the Red Sea as we pass from death into life, all during the thanksgiving prayer over the waters.

C.S. Lewis also places heaven over the sea in The Voyage of the Dawn Treader, and across space (or at least a Garden of Eden on Venus) in Perelandra, by the way.

In short, the water itself is a metaphor both for the death of sin and the passing into new life. Whether it is a river or the Red Sea is just a matter of breadth.