I've driven the coast of Croatia and would recommend staying in Trogir, which is just a short distance from Split, where the big attraction is the Diocletian's Palace. From Split, you can take a ferry to the island of Hvar. I've been to about half a dozen islands off the coast of Croatia, and my favorite by far was Hvar. A lot of the islands have very little on them - one restaurant perhaps - but Hvar has galleries, restaurants galore, beaches and is simply very attractive. From Split, you can continue your drive to Dubrovnik.

I would plan on a full day for Trogir/Split and another full day for Dubrovnik.

As far as a boat charter, are you thinking power or sail? Bareboat or crewed? I did a bareboat sailing charter out of Dubrovnik. There are also charters out of Trogir, and I wish I would have gone out of there as the islands further north are more interesting. You could certainly charter a boat out of Trogir and head down to Dubrovnik that way and avoid driving. We did spend a week sailing from Dubrovnik to Hvar and back, but you could certainly do it faster, especially in a power boat. If you have questions about that, feel free to drop me a PM.

If you go inland at all on your trip, Plitvice Park in Croatia is very beautiful and well worth a visit. We drove from there to Trogir.

Here's a trip report from when I was in Croatia in 2004. (Although I just reread it and realized I ended it about half way through the trip. )
http://www.flyertalk.com/forum/showt...=333512&page=1

and here are some photos (you might have to refresh it to get them to all load - there should be about 35). The first few from Plitvice and there are some from Hvar, Torgir, Split and Dubrovnik, among other places.