The ship in Dubrovnik will be docked either in Gruž harbour (Luka Dubrovnik) or under Franjo Tuđman bridge, a bit further - it's about a 10 min ride from the Old town. If the ship is in Gruž harbour, it will take you some 20 min to walk to the Old town. If you embark in Dubrovnik, please call the contact number given to you for the embarkation day and ask where exactly your ship is docked.