Before the Changing of the World, anyone could get to Valinor by simply sailing west. Even the hostile, invading army of Numenor landed there without much difficulty, it seems.
Then Eru stepped in, and...
...responded by creating an enormous rift between Númenor and the Blessed Realm, into which Ar-Pharazôn's fleet and, ultimately, his land were swept. Meanwhile, the flat world bent back upon itself and met at this rift. The final result was a round planet of which Aman and Tol Eressëa were no longer a physical part. After the Changing of the World, only the Elves could find the Straight Way and reach the ancient West by ship.
So what does that mean for Men sailing west in the Third Age? Would they just pass innocently through the "rift" that the Elves could penetrate, sail around the world like Magellan and land on the far eastern shore of Middle Earth without noticing anything was amiss? Or is there a danger to sailing that course, some risk to approaching that rift if you are not an Elf (or being escorted by one, like Frodo)?
Obviously Middle Earth is supposed to become our world, so at some point any discernible "border" would have disappeared, but given that Third Age maps of Arda are still radically different from modern Earth, such a "seam in the world" could easily have been removed once all the Elves had left.
Do we ever get any indication about the ability of Men to sail west? Were they forbidden, or cautioned not to attempt the voyage, as part of the punishment for Numenor's indiscretions? Or would they just pass harmlessly over the waves, oblivious to the experiences of their Elven counterparts?