This achievement requires you to use a ship to travel from England to mainland Europe (or vice versa).
There are a number of ports in the game, including Newcastle-upon-Tyne (UK), Grimsby (UK), Felixstowe (UK), Dover (UK), Calais (France), Rotterdam (Netherlands), and Amsterdam (Netherlands).
On the world map, each port is linked to at least one other port via a dashed line.
You can use these ports during an international mission, or alternatively, during free roam; though, you must own your own truck.