Ocean
An Ocean is yer Piratey world of fun. The Oceans are distinct and separate, and no trade is possible (well, it is, but don't do it or the OceanMasters'll do it in for ye) between them. They are:
Subscriber Oceans
Midnight is a subscriber ocean, the oldest still live.
Cobalt is another subscriber ocean. It is a clone of Viridian; the only (artificial) differences are the name and the subscriber vs. Doubloon payment method.
Doubloon Oceans
Viridian is the only Doubloon ocean as of yet, and it is a 'clone' of Cobalt. It is different however in that it is a Doubloon ocean rather than subscription.
Ice, the Test Ocean
Ice is a 'test ocean' which requires a separate client to play. It has surprisingly few players on it, despite the fact that new features are released there faster than the other oceans. Ice was opened not long before Cobalt and Viridian.
Where'd they go?
Azure was the first ocean. Then Captain Cleaver went crazy one day and sliced the server up with his famous cleaver and ate it for breakfast. (Disclaimer: don't do this.)
There was a Chinese ocean once. There isn't now. (Hmm...)