We are planning to drive from Dubrovnik to Split and then take a car ferry over to Hvar and base there for a few days. The question is do we book the car ferrry online before we go.. or book it when we are in Dubrovnik? Or just do it on the same day at the ferry?
Croatia planning is far more challenging then other European countries isn't it!
Jabo

Jabo, in my experience, you cannot book the ferry in advance. Jadrolinija is Croatia's main ferry line; that being said, it has been traditionally difficult to get information about the ferry schedule and tickets on-line. But I just went on this page and it now appears that there is much more forthcoming information. It appears that there are local car ferries that run from Split to Hvar, but you cannot book them on-line. Then there are coastal car ferries, that you can book on-line. Pay close attention to the days that they sail - some are not so regular, more sparsely scheduled. Here is the coastal car ferry schedule.

When we went, we did not bring our car; we went on the passenger ferry, and we had to book that the same day. That may be changing now, not sure.

A photo of Split harbor from the ferry:

We took a catamaran - Krilo Express- between Hvar and Split. It was about a half hour faster than the ferry. (and perhaps marginally more costly - all very reasonable - maybe $10 each) You cannot purchase those tickets online, but instead at local travel agency in Hvar. No problem getting tickets on either the ferry or the catamaran - you can't buy them till the day of travel. I am kind of compulsive and like to have things ahead, in hand. But there really wasn't anything to worry about, at least not during our September trip. Just get in line in a timely way, or get up early and buy your tickets first thing in the morning. If you are taking a car, the situation might be different - I can't report about that, as we didn't have a car with us.

We took a bus between Split and Dubrovnik. Again, couldn't buy ticket ahead....and again, no problem. It was about a 4 hour trip - quite scenic. Very inexpensive.
I do think you'd want a car on Hvar if you stay for more than a day. We didn't have a successful car rental experience there (see my trip report) but I'm sure there's a way, perhaps involving bringing one over yourself from Split (seemed like a hassle to me, but maybe not)

Linda