Because Greece was so mountainous, and every place in Greece was so close to the sea, a lot of people in Greece used boats to get from one place to another, even starting as early as the Stone Age, when people sailed to the islands to get obsidian, and also went out in boats to catch tuna fish.
Greek people used boats to fish, and to trade with other places, and also to sail to other cities and fight them and take their stuff. Greek sailors had different kinds of boats for all these things – small rowboats for fishing, and big trading ships, and fast warships. By the 600s BC, the Greeks were among the best sailors in the Mediterranean Sea (along with the Phoenicians).
Like other people at this time, Greek ship-builders built their ships from the outside in, first the hull and then the insides. They used only one big square sail, made of coarse linen cloth, though Aristotle seems to say that by his time, in the 300s BC, sometimes sailors squeezed down one side of the sail, the way you can squeeze one end of your window blinds, in order to catch the wind. When even that didn’t catch enough wind, Greek captains forced slaves to row their ships with wooden oars.
Even though the classical Greeks were great sailors, they didn’t like to go out of sight of land if they could help it, because they didn’t have compasses or astrolabes or any way to tell where they were if they couldn’t see land. Then about 140 BC, just as the Romans were conquering Greece, Hipparchus of Rhodes invented the astrolabe.