We are wanting to visit the Croatian coast in Sept or Oct. We would like to sail from Rovinj to Split to Hvar to Dubrovnik and then probably head back to Italy where we will start the trip in Venice. Are there high speed ferries? Small cruises? Also how many days should we stay in each place. Thanks.

You could start with this website for ideas -

http://www.jadrolinija.hr/default.aspx?lang=2

We have been to Croatia twice and loved it, but have not used the ferries. First trip was just Dubrovnik, the second was Rovinj and Opatija and also Piran, in Slovenia. I would say 3 nights minimum in each place, that only gives you two full days. I'm pretty sure there are small cruises that go along the Croatian coast, you could try a google search.

Kay

The site listed by Kay is a good start. But you'll see that while there are ferries doing all those routes, they are sometimes inconvenient times. So you'll need several days in each location to make it work. We took a ferry from Dubrovnik to Korcula (which is close to Hvar) and then from there to Split. It's perfectly doable, but even in July there were not ferries every few hours or anything, it took a little bit of coordinating. But Croatia was beautiful and the ferries a great way to get from place to place. And you want at least two or three days in each place anyway. We had four Dubrovnik, two Korcula and two in Split. You might be interested in my trip report - http://www.fodors.com/community/euro...rful-weeks.cfm

I have been thinking of doing a similar trip around the same time but my research revealed that the number of flights/ferries into the area and local ferries drops off sharply once September arrives. I would be interested to hear what your experience is.

There are certainly small cruise lines. The ferry schedule may not be up to date, but you can extrapolate from old schedules.
In light of the fact that you want to end up in Venice, I suggest starting in Dubrovnik and moving north. Once in Rovinj, you can travel across Istria and then take a ferry to Venice.
How much time do you have?

But does the Venezia Lines boat from Rovinj to Venice run in October? In 2011 it only starts in mid-April.

To the OP, the later in the season the fewer transportation options. In even in high season, not every boat runs every day. You have to pore over those ferry schedules very carefully.

Good point Mimar, the OP will have to check ferry schedules. Th alternative is the Dubrovnik-Ancona or Split-Bari ferry, but I am not sure if they are year-round either.
Until we know how long they have, it is difficult to make itinerary suggestions, but there is always the train or bus from Istria.

Thanks for all the replies. We will have about 8 days. I like the idea of starting in south and moving northward. I will definitely start looking at those ferry schedules. Hopefully I can find last years to review. We could rent a car but I really love the idea of sailing up the coast as it is so beautiful from everything I have seen. There are many large cruise lines who do the Croatian coast but that just isn't us.

With only 8 days, and if you have the budget, I would look into the small cruises. I don't know anything about them, I have just seen some lovely small ships docked at Trogir.
Alternatively, you'll have to determine the day that week that the Jadrolinija coastal line is running and try to plan accordingly. For example, a couple days in Rovinj, then the overnight + ferry, then disembark at Split, and you can make your way to Hvar from there easily. However, you may have to return to Split and take a bus to Dubrovnik in lieu of another ferry.

Most of the small sailing vessels do not sail the entire coast. They have preset itineraries which are either southern, central or northern coast and many are 7 day trips. With only 8 days you do not have enough time to get from Dubrovnik to Italy by boat. The later in the season, beginning in October our ferry schedule begins to change drastically, especially if our weather turns windy. If your dates are set, Jadrolinija ferry may or may not work for you. The views from the bus trip along the coast are magnificent, often better than from the sea. Enjoy your time here.

Heading north from Dubrovnik, there is another (private) ferry operator that operates a small fleet of 4 fast catamarans. Check on G&V line http://www.gv-line.hr/index.php?lang=E. They operate throughout the year with extended sailings in the summer months when they link up Dubrovnik with Korcula and the island of Lastovo. The Korcula run can also be done with the local bus up the Peljesac peninsula and the short ferry across from Orebic to Korcula Island.
With Jadrolinija, also remember to check the timetables rather than the map search feature - tends to be more accurate. Also remember they also have local car and passenger ferries on the same routes. As these ferries coming out of Dubrovnik going north are commuter runs, they run the whole year.