Instructions
Swim to the pool under the bridge in the Pacifica overworld map. This is where the boss Barbos is located.
Defeat the boss Barbos by dodging her attacks and waiting until the shells separate to expose the center of the creature. This will reveal Kaos Kore on the world map.
Leave Pacifica and go down at the first flag you see. Mount the jetski and ride it to the fortress under the bridge that has newly opened up. This is Kaos Kore.