Boat and Ferry
Several companies run ferries between Italy and Croatia throughout the summer, the largest being the national carrier, Jadrolinija. The most popular route is Ancona to Split (11 hours), with up to eight crossings weekly in the summer (twice weekly in the low season) from Jadrolinija and the Italian-based company SNAV. The Ancona to Zadar ferry (9 hours) travels five times weekly in the summer (no service in the low season), while further south, there is a daily ferry between Bari and Dubrovnik (11 hours). Up the coast, Trieste Lines operates a high-speed ferry in the summer between Trieste and ports in Istria, including Rovinj (1 hour, 30 minutes), Pula (2 hours, 15 minutes), and Porec (1 hour), while Venezia Lines and Adriatic Lines connect those same Istrian ports with Venice (3–4 hours).
If you're traveling with a car or other vehicle, you only have three options: the route from Ancona-Split, Ancona-Zadar, or Bari-Dubrovnik.
Within Croatia, regular ferries connect all islands to the mainland, and slightly fewer connect the islands to each other. Jadrolinija operates all car ferries, while a few private companies operate passenger ferries. Keep in mind that local ferries are quick and cheap for foot passengers (you can usually show up at the port, buy a ticket, and jump onboard), whereas car passengers should reserve in advance, and still might end up waiting hours for a spot in high season.