Since Greece has been extensively covered by mountains, and every place close to the sea, the Ancient Greek sailors (especially the Corinthians, due of their unique position for trade and colonization) used boats to get from one place to another, so they were expert shipbuilders.
However, their experience in the arts of navigation, was obtained from the Egyptians, Phoenicians and the Carthaginians which as early as 600 b.c. were already able to have built efficient vessels such as the triremes.