Nobody knows much yet about the people of Greece in the Middle Stone Age – the Mesolithic (approximately 9000-7000 BC). People still lived mainly from gathering and hunting and fishing, especially for tuna fish. They still did not farm or use metal or build houses. They lived in caves.
Somebody from Mesolithic Greece seems to have sailed on the Mediterranean Sea in small boats made of reeds and animal skins. People in Mesolithic Greece would probably have needed boats to catch tuna, and also they must have sailed because they used tools made of obsidian. You can’t get obsidian on the Greek mainland, only on the volcanic islands nearby.
The nearest place where there is obsidian is on the Aegean islands between Greece and Turkey. So either the mainland Greeks were sailing to the islands to get obsidian, or the islanders were sailing to Greece to sell it to them.
Learn by doing: go to a rock shop and check out some obsidian
More about the Neolithic in Greece
The Stone Age: What Life Was Like for the Earliest Humans, by Philip Steele (2000). Includes instructions for projects.
Eyewitness: Early Humans, by Nick Merriman (2000). With great pictures.
The Archaeology of Greece: An Introduction, by William Biers (revised edition 1996)Biers writes very clearly and has a lot of good pictures.
The Early Neolithic in Greece : The First Farming Communities in Europe (Cambridge World Archaeology), by Catherine Perlès and Norman Yoffee (Editor) (2001).