How on earth did the Númenóreans become such good mariners?
“Above all arts,” says the Akallabêth, the Men of Númenor “nourished ship-building and sea-craft, and they became mariners whose like shall never be again since the world was diminished; and voyaging upon the wide seas was the chief feat and adventure of their hardy men in the gallant days of their youth.” With the exception of the Undying Lands, travel to which was banned, the Dúnedain traversed the Sundering Sea and beyond: “from the darkness of the North to the heats of the South, and beyond the South to the Nether Darkness; and they came even into the inner seas, and sailed about Middle-earth and glimpsed from their high prows the Gates of Morning in the East.” In other words: they got around.
To travel the world like that doesn’t just require hardy seafarers and ships, it requires skilled navigation. And that’s where the problem is. Before the Changing of the World that destroyed Númenor bent the seas and made the world round, the world—Arda—was flat. And if you know enough about maps, navigation, or mucking about with boats, you know that will have serious implications for navigation.