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

There are etchings theologically speaking of Heaven being across the sea - specifically the Crystal Sea. For example, Revelation 4:6 and 15:2 speak of the saints standing on the sea that is as smooth as crystal (e.g., Crystal Sea). Based on the context, the two different groups of saints are standing before the Almighty's glorious throne on the Crystal Sea. This occurs in the Outer Court of Heaven (note Ezek 10:5), prior to them being glorified and entering Heaven's wondrous Inner Court that contains the New Jerusalem. Much more could be said about our passage across the sea. Please check out Our First 22 Days in Heaven.

Of note is that the sea symbolizes several things for the Jewish people in antiquity, most notably disaster or ruin. For the most part, the ancient Jews were not seafaring people. Deuteronomy 28:68 gives them a stern warning against disobedience, clearly stating that disobedience would ultimately lead to a re-enslavement with the notable inclusion of being sent back in ships ("And the LORD will bring you back in ships to Egypt, a journey that I promised that you should never make again; and there you shall offer yourselves for sale to your enemies as male and female slaves, but there will be no buyer.”)

The waters/sea are also present in the Deluge, the Exodus, Jonah's adventure, etc. The Psalmist also speaks of it with a fearful reverence in Psalm 107:23-32.

Other notable verses state:

"The wicked are like the troubled sea, when it cannot rest" (Isaiah 57:20).

"There is sorrow on the sea; it cannot be quiet" (Jeremiah 49:23).

Most appropriately to the topic at hand, though, is that the sea is also a symbol of complete and total separation, something which is notably present in the writings of John the Apostle in the book of Revelation, Ch. 21. In 21:1, you read "...the first heaven and the first earth had passed away, and there was no longer any sea." Now that's an interesting phrase. No longer any sea, John? Why would you make note of that?

John's peculiar circumstance led to the development of a particular disdain towards the sea, which the text clearly demonstrates. He was in exile on the Island of Patmos; twas a barren, empty island. Finding himself amid a proverbially damning sea, and cut off by its waves from all that which he loved best. How often do you think he would turn his gaze and helplessly look towards his own beloved land of Palestine? What wonder, then, that he would have constant thoughts and visions of making it home (that is, heaven), and in doing so, is it any wonder that he should say, "And there was no more sea"? For the sea, the dreaded sea, represented separation. And in crossing that chasm, John would finally find himself in heaven.

Thus, we see a clear source from which these writings and others may draw from when they present heaven, a place of joy and repose, found across that pesky, troublesome sea.

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?

When describing the paradise after judgment day in Revelation 21, the Revelator begins (ESV quoted):

Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more.

If you think of seas as separating worlds--as the "Old World" of Europe is separated from the "New World" of America--then you can see this as saying that the sea that is done away with here is an image of the sea of everything that separates us from God's country.

Aslan's Country is not Heaven - it is Paradise, it is important to read carefully The Last Battle if you want to make a comment on this part in particular. Aslan's country is not unconnected with heaven, as Eden of old and Paradise in concept are a type of Heaven. But if you pay attention to The Last Battle, Aslan's Country is itself only a figure -- Heaven is 'inward and upward' - far beyond Aslan's Country.

As for why the image is chosen - across the sea - it is because it is traditional and mythological. It is a tangible symbol of the passage of death, especially to the landbound ancients, and even those that sailed. And we have the connection of baptism with death and resurrection, unsurprising as baptism as a word comes etymologically from the term used when ships were sunk.

For Tolkien, I'm certain he's borrowing from old myths and not making an analogy at all. He himself stated this, so that if there is a reason why this is an image of paradise or heaven it is only because people throughout the ages thought it so, not because Tolkien decided to foist that image upon his reader.

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.