From what I recall they thought that East Asia was on the other side of the atlantic (as they where thinking of the world in being exclusively comprised of Eurasia and Africa, like on the Ptolemean maps) and Japan (Cipangu) was the first island you could reach from the Atlantic coast (except for the mystical island of Antillia who was supposed to lie down in the middle of the Atlantic and the Canaries and Azores who are quite close to the known european coasts). I think the maps who would describe the most accurately the pre-exploration era vision of the Atlantic would be the ones made by Paolo dal Pozzo Toscanelli, a Florentine astronomer.