I'll need more time to ponder this interesting question,
lmp, but my first spontaneous thoughts are these: The Sea is the passage-way from one to the other. The Elven mariners in
Smith come over the Sea, the Sea separates Middle-earth from Valinor, which can be reached only by finding the Straight Path, so could we speculate that there is another Straight Path somewhere in the Sea between Valinor and our Earth? Of course, according to Tolkien's original plan, Middle-earth
is our earth, only very long ago or in another time dimension. That raises the question of time travelling by Sea, perhaps?