Hi Kevin,
I was on holidays in Croatia in June 2010 but the fishing was very poor.
The last week of the trip we spent on one of the islands (Pasman) staying in a fisherman's cottage and went out with a local guy in his boat but fishing was very bad and i wouldn't recommend that guy not least beacuse of one particualar incident, I caught a female cookoo wrasse, he grabbed it off my hook and whacked it over the head before I had a chance to stop him. Then told me that it was the first time he had seen a cookoo wrasse in 5 or 6 years and thought they were extinct in Croatian waters
You could try these guys:
http://www.game-fishing-adriatic.com/However, August is quite late, Spring, particularly April is supposed to be best time to fish, and later in the year when temperatures begin to cool down.
Good Luck