The best option is to take the ferry from Zadar to Pula and change there for a bus to Rovinj.
As we are still waiting for the schedule confirmation of the Zadar to Pula ferry for this year but it should be similar to the previous one:
https://www.croatiaferries.com/zadar-pula-ferry.htm
The ferry as well as the bus ride itself is very scenic and comfortable.
I expect the whole bus journey will take about 6 hours. The total price – about 25 to 30 euro
Map with all stops on the way>>>