I have a few friends who did charter in Croatia. If you have any specific questions - I can put you in touch with them. In general it appeared to be friendly, not particularly difficult and a good location.
One thing to consider - like most european countries, they require a "licensed captain" to charter. This means, that for US residents they want to see a USCG license - which is not too common. ASA certificates don't really do it for them. Larger charter companies (Moorings) have some sort of a deal with the government, where they can issue a "temporary license" of some sort based on experience, but from my conversation with them it seemed to be an uncertain way to go, potentially not available anymore. So, unless you have USCG license, or something official from another country - chartering there may be difficult, or you may be forced to hire a captain.