It was nearly 200 years ago when the Red King declared that he was no longer evil and began calling the Dogwa his enemy. Until then, the sea between the Redlands and the Dark Domain was held in an iron grip by the navies of each nation. During the Redemption War (as they call it in this part of the world) the Red King broke the power of the Dogwa's navy and freed most of the island nations that made up the Archipelago.
While most of the newly freed city-states remained friendly to one nation or the other, they weren't neccesarily friendly with each other. Petty border wars plagued the next century until finally settling into the political climate we have today: hundreds of island nations vying for economic control of the many shipping lanes to the Redlands, Dark Domain, and the Whitelands.
If you hadn't guessed already, the whole sea is a haven for pirates. Many city-states boast a "navy" that's little more than a few barely controlled privateers, while others pay a "protection" fee to one band of scalawags or another. Some nations are even cities sprung up around strongholds of pirates that have enjoyed a successful career and decided to retire.
It's called "uncharted" because no matter how well a sailor knows the waters, there's always something that will surprise him. Mysterious islands that appear where there was only deep water before, whirlpools, sea monsters, coral reefs, and changing currents are only a few of the factors that will send an unwary mariner to Davy Jone's locker.