You have to take the person from Canouan to the principality that's located on the other side of the map. I don't recall the name now sorry. That principality is right next to a pirate hideout. He's going to help you read some documents and to tell you that France is gathering some fleet on the northern principality of the map and you'll have to stop it. Take your corsair to the first one and at the same time order a fleet to north. Don't drop the anchor. Just block the acess by defeating/destroying all the french ships.