Random quest obtained as rumor from tavern keeper, shopkeeper or people in church and jungle.
First you must get a rumor about a ship sailing with certain cargo. I think you'd get dialogue message that's something like this: "There's a ship named <ship name> sailing under the flag of <nation> from <colony> carrying x amount of <cargo>." After that log book will update with info.
Next you must sail out of the city, get to the world map, and just sail to the first ship you see sailing under that nation's flag. After you find the ship, board it or sink it and log book will update, and quest will be over. You can also ignore it and after a while log book will update.
Based on original from Skodarap's CoAS Site (used with permission). Original is here .