Ships and ferries operate along the coast and to the islands. Express coastal lines start from Rijeka and call at many towns and islands all the way to Dubrovnik.
All ferries to Croatia originate in Italy but the schedules vary by season.
There are regular and seasonal services to Italian ports of call (Bari, Ancona, Vasto, Pescara, Giulianova, Venice, Grado, and Trieste) and Slovenian Koper. Aboard the ships and ferries operating along the coast and across the Adriatic to Italian ports (except aboard the fast ships and catamarans) there are restaurants and you can either reserve a cabin or travel on the deck in reclining seats.
Naturally, there are far more ferries between Italy and Croatia in the summer. Most of the expanded services start around April or May but some are July and August only. The season ends in September or early October. In the tourist season there are also many additional tourist lines and excursions from/to the mainland and island destinations and between Croatian and Italian ports.
Many Croatian travel agencies have their own boats and organise ship transfers and excursions by from various destinations.