Reaching Croatia by ferry is relatively easy as long as you're travelling from Italy.
No ferry service exist from or to any other country (for example Greece, Montenegro or Turkey). The only international ferry route to Dubrovnik is the one from the port of Bari (Puglia region in Italy). This service is seasonal (from June until end of November) with frequent sailings during the peak season months (July and August).
The journey on this route is done by car/passenger ferry "Dubrovnik" which has a capacity for up to 1300 people and around 300 cars. The ferry features 149 cabins which comes in range of 2, 3, 4 berth (inside and outside).
Onboard the ship you will find self-service restaurant with 224 seats, patisserie, coffee bar, video room, movie theatre, chapel, children's play area, a la carte restaurants and duty free shop.