In Dubrovnik, two cities, and two ports. It has the old town (a UNESCO world heritage site) and the modern city (where the majority of people live). In the old town there is a fishing port, and the modern, a port for ferries that take you to the surrounding islands. Sometimes they call them "catamarans" but they are just smaller boats that don't carry cars, only people. Then there are ferries, for example Jadrolinija, which take you more slowly but allow cars. In general, if you are on foot, the prices are pretty cheap, you pay 1 or 2 euros per hour of travel. Some of the islands near Dubrovnik which you can go to during the day are Lopud, Mljet, Korcula, Kolocep and Sipan.