The Ancients were not that primitive. There is the Periplus Maris Erythraei, a manual for traders that describes the ports of the Indian Ocean, from Zanzibar to Sri Lanka. These are the ports that were regularly visited by merchantmen on trading voyages. More adventurous characters might want to cut out the middleman and sail further to China, whose existence is mentioned in the same text.

700 years prior Pharao Necho II outfitted an expedition to sail around the African continent, for no other reason than research.

http://www.livius.org/sources/content/herodotus/herodotus-on...