The South Seas were originally planned to be the setting for World of Warcraft's first expansion. Johnathan Staats describes it as having had a flavor similar to Booty Bay and Stranglethorn Vale and explains that the development team were extremely enthusiastic about pirate themes following the release of the Pirates of the Caribbean film. However, technical limitations meant that the servers were getting too overloaded with data in Azeroth and its continents of Eastern Kingdoms and Kalimdor, preventing Blizzard from just adding more zones without either upgrading the server equipment or making the code more efficient. Thus, the team decided not to add more Azeroth content and instead use another realm, Outland, as the setting for the first expansion, later named The Burning Crusade. This is also why the Azuremyst Isles and Quel'Thalas are on the Outland realm instead on their proper continents.
The South Seas supposedly boast action, unique civilizations, and ancient mysteries. On the negative side, most of the action is swashbuckling, most of the unique civilizations are composed of bastards like the naga, and most of the ancient mysteries are dangerous. It is possible to cross the South Seas with a gryphon as that is what Brann Bronzebeard did after purchasing Gryphadin in Steamwheedle Port.
10,000 years ago all the lands in Azeroth were part of the same giant landmass: the primeval Kalimdor. During the War of the Ancients, Queen Azshara summoned the Burning Legion with the Well of Eternity's energy, and at the climax conflict, the well imploded and tore the giant continent asunder, splitting it into the continents as they exist today... and a few islands. It also created a raging magical vortex known as the Maelstrom, which occupies the center of the Great Sea. This vortex forms the northern boundary of the South Seas, and south of it are several tropical islands. Though small, these islands have played pivotal roles in history.
Kezan is the largest and southernmost of the South Seas islands, and, despite the goblin presence, the safest.Bilgewater Port is the port in the South Seas, and the only one worth mentioning that exists between the western and eastern continents. Of the South Seas islands, Plunder Isle is the closest to the mainland, in this case to southern Eastern Kingdoms.
Although the South Seas are not mentioned, the island of Hiji appears in the Warcraft: The Roleplaying Game book. It seemingly takes the place of Plunder Isle as both are located in the same area and roughly have the same form.
The South Seas borders Pandaria, including the southern regions. This shouldn't be lorewise as the South Seas are part of the Great Sea thus the waters south of Pandaria should be a different sea. A possibility could be the Uncharted Sea.