This is a place to argue for Progressive Barbarians and other such Mechanics. Wherever there was a coast conducive to piracy - Fat Target trade routes passing by and mountains or desert behind the coast to make it difficult for any central Authority to control them - groups made a living out of piracy, and became specialist pirate states. They developed, in fact, into something bigger than simple Barbarian Camps and more, in game terms, similar to Pirate City States. This goes as far back as the Dalmatian and Anatolian coastal pirates of the Classical Era, or the Barbary Coast of North Africa, the south China coast or, of course, Port Royale in the Caribbean, probably the quintessential Pirate City State in history.
Basically, the Pirate City State would be a coastal version of the Military City State which produces Sea Raider type ships almost exclusively, is much harder to get rid of than a simple Barbarian Camp, but can be interacted with Diplomatically as well aas Militarily.
It's another argument for more Dynamics in the game: the Barbarian Camp that spawns 'pirate' ships, if it keeps at it long enough, should grow into something bigger and a little more dangerous, just as I would love to see Free Cities have a chance of becoming a New Civilization to keep the mid and end game more dynamic and less predictable.