[FredZarguna]

The metaphor is intended in the sense that, just as the ocean has the earth as its shore, the ocean of night must crash on the entrance to the underworld [The domain of Zeus' brother, Pluto.]

It's a beautiful image, not improved by an astronomy of which Poe could not have been aware.

[Verginius Rufus]

Thanks for the explication of the metaphor.

Not only was Poe unaware of the existence of the planet (sic) Pluto, when he wrote the poem Neptune had not yet been discovered. "The Raven" was first published in January 1845, and Neptune was discovered in 1846.