The Ancient Greeks used the sea for many things just as societies have throughout history.  First, they used the sea as a source of food.  They caught fish in the sea for their food.  Second, they used the sea as a means of trading.  They used it to give them the ability to buy from and sell to other cultures around the Mediterranean.  Finally, they used the sea for war.  They eventually built a navy that came to be an integral part of their defense, serving to help them win (notably at the Battle of Salamis) the war when the Persians invaded them.