Won't not apply to you but in the off-season (pretty much beginning of October until sometime in spring), a lot of the coastal ferries stop working and this is something that started only in 2010. For example, the car ferry from Rijeka in the north that connects to Bari via Split and other places stops (only does Bari-Dubrovnik). There's also no Dubrovnik-Korcula ferry either.

Plan well ahead (but recheck closer to the time!) as the sailings don't always operate daily.

Last autumn, my wife and I were planning to take the overnight ferry from Rijeka to Split, then to Hvar, Korcula, Dubrovnik and Bari, renting a car only in Dubrovnik for day trips. At the last minute (well, about 4-8 weeks before we left Canada), the Rijeka-Split ferry sailing disappeared (part of the car ferry that goes to Bari) and so did the Korcula-Dubrovnik ferry. We had to rent a car in Opatija and drive to Split (but that gave us a chance to see Istria), and get a shuttle for Korcula-Dubrovnik.

The Split-Hvar-Korcula legs (foot pax only sailings) were on fast cats and unfortunately you're in the enclosed cabin for these sailings.

The drive between Rijeka and Split isn't very interesting either (there was one city we had to miss due to lack of time but we wouldn't have seen it if we ferried).

Not sure if the car ferry seasonal curtailment is due to all the fast toll roads.