You can use the port city coordinates shown in the Plunder Goods Warehouse to Set Sail. 

Press the Set Sail next to the coordinates shown in the Market Price Info tab or press on the coordinates to directly see the Port on the Sea. You can then press the Set Sail button that appears upon clicking the Port.

■ How to Set Sail to Port Cities

- [Main screen] > [My Territory] > [Plunder Goods Warehouse]

> [Trade Info] > [Market Price Info] > [Set Sail]