Croatia has 5000 km of coastline and more than 1000 islands so sea transport is quite a common form of travel here. A large number of ferries and high-speed boats (catamarans and hydrofoils) connect islands with land.
Some of them are available through the whole year and some operate only in high season. So be sure to consult the right timetable when planning your trip. National ferry company is Jadrolinija, and several other private ones operate alongside it.
Coastal ferry route carried out by Jadrolinija runs the whole length of the coast: Rijeka-Split-Hvar (Stari Grad)-Korčula-Mljet (Sobra)-Dubrovnik. It's a 20 hours journey (with one night aboard).
*Tickets can be booked online
Local ferries and catamarans connect major ports with most of the islands. Tickets can be booked online.
*If you plan on bringing your car on board keep in mind summer crowds can be pretty thick. In summer, especially on weekends, wait time to board can last a few hours so it's recommended you come to the port as early as possible.
For more info on timetables and routes check these websites: