Ferries IN Croatia connect the Croatian coast with all inhabited islands. Ferries TO Croatia connect Croatian ports with Italy. (See more about ferries TO Croatia)
Jadrolinija is the state-owned company that runs nearly all ferries in Croatia, helped by a few private ferry companies. Local Jadrolinija ferries connect all islands to the mainland and there is a summer coastal ferry from Rijeka to Dubrovnik. Ferry services from one island to another are much spottier. Dont assume that there "must be" a ferry from one island to another. Service is year-round on most routes although a few are summer only. Ferries are generally much more frequent in the summer.
Reserving Local Ferries
Passengers can always get on board but reserve ahead if youre bringing a car, especially in the busy summer season. Unfortunately you cannot reserve local ferries online; You reserve most local ferries at a Jadrolinija or at a local travel agency. (See more about booking local and coastal ferries in Croatia). The only time you cant reserve ahead are on the shuttle car ferries that connect the mainland with several island ports.
You cannot reserve in advance on the following lines; you must buy tickets on the spot:
Drvenik to Sucuraj on Hvar Island
Valbiska (Krk Island) to Merag (Cres)
Jablanac to Misnjak (Rab Island)
Ploce to Trpanj (Peljesac peninsula)
Orebic to Domince (Korcula Island)
Show up at least 1/2 hour in advance (1 1/2 hours in the summer), park your car in the queue, collect your ticket from the nearby stall and wait. On the above routes, ferries run from four to eight times a day, depending on the season, and cost about 80HRK for a car and driver for the 30-minute run. See more about booking local and coastal ferries in Croatia.
About local ferry schedules
Check the local ferry schedule carefully! Many routes from the coast to the islands are designed to bring islanders to coastal cities to work which means that they leave the island early in the morning and return late afternoon. Thats exactly the opposite of what tourists are looking for to make a day trip to an island.