Why do we assume that Atlantis is to be found in the Atlantic Ocean? The Atlantic Ocean as we know it today, was named by the Romans for Mount Atlas. This was, thus, well after Plato and his directions. And Plato could not have known of this "renaming".
The "true ocean" per Plato, was the Atlantic, which is any ocean outside the Mediterranean. And "beyond the pillars of Heracles" or the straits of Gibralter, would mean anything beyond.
Check out the following link...
Certainly worth a peek...