This achievement is easiest with just the base game, as many if the map expansions add additional "ports". You need to travel to or from every "port" in the game. In the base game this includes the English Channel ferries (4 ports in the UK, 2 in France, 1 in the Netherlands), the Train under the Channel betwen Dover in the United Kingdom and Calais in France, and the River crossing in Travemünde in the northern part of Germany. Using a Seaport and the Train for the first time will each unlock an achievement of their own as well.