The Greeks get credit for inventing a lot of things, but did you know one of them is piracy?
There've been pirates in the Aegean Sea for at least three thousand years. The Aegean is ideal for this sort of thing. There are a zillion islands, the coastlines are rocky, and the only way for trade to move about is by sea.
The world's first known pirate was a Greek: a guy called Piyamaradus. He's mentioned in a tablet written by the King of the Hittites to the King of Mycenae. That tablet has to be older than 1,200 BC! The Hittite king complains that Piyamaradus has been using the Greek city Miletus, in Ionia, as a base to raid Hittite towns, and he expects the King of Mykenae to do something about it. The Mykenaeans agreed to hand over the pirate, but Piyamaradus escaped, no doubt in a daring escapade.
Pirates were still a major problem more than 1,100 years later. It's a well known story that the young Julius Caesar was captured by pirates from Cilicia, which lies along the Aegean, and he had to be ransomed. In fact, when he discovered how much they were asking for him, Caesar was offended and demanded the pirates more than double their price. While they waited for the ransom, Caesar hung out with his captors and joked to them that when he was free he'd return and kill them all. Caesar was duly ransomed, at which point he returned and killed them all.
The Roman Senate eventually sent Pompey to clear out all the pirates. Pompey offered them lots of money to go away. The pirates took the money, and then ten years later they were back.
Pretty much every ship on the sea was a potential pirate if a safe opportunity arose. If a larger merchant ship came across a smaller, much weaker one from another city, why not take their cargo? If it led to fighting, well, tough luck for the small guy.
Nor was piracy necessarily frowned upon; in archaic times it was an acceptable way for a noble chap to behave. Mary Renault in The Bull From The Sea has her hero King Theseus spend time raiding merchant ships after he's tossed out as king of Athens. In the Odyssey, Homer has characters boasting of their skill at piracy.
Pirates made their money in the traditional way by stealing from other ships. But they also exacted tribute from nearby towns in return for not raiding them, and raided towns that didn't feel like paying. Pirates took any rich men they came across for ransom, and they collected innocent victims to sell as slaves. It's said that Plato himself was once captured in a pirate attack when he travelled overseas. They sold Plato as a slave, and his friends had to buy him back.
By classical times there were highly organized pirates with for-real pirate bases. I'm afraid you'll have to do away with ideas of eye patches, peg legs and parrots screeching for pieces of eight. Your average pirate looked like any other sailor. Successful pirate chiefs became wealthy and could marry well.
I suspect one of the reasons the island states stuck with Athens, even through the hard times, was that the Athenian fleet could suppress piracy. The Athenians ripped off their client cities something chronic; so it was totally in their interest to keep trade flowing to increase their own takings, and when you've got 300 triremes to work with, you can afford to assign a few to patrolling trade routes.
No pirate in his right mind would have taken on a trireme. The most powerful pirate ship would have been a pentekonter at most. As the name suggests, a pentekonter has 50 oars arranged in a single row (pente: 50). But even a pentekonter is expensive; the vast majority of pirate ships would have been converted merchantmen.
Even the word "pirate" comes from Ancient Greek. When he wrote about pirates, Plutarch uses the word peiratiko (πειρατικo) to refer to a pirate.