From Zurich to Rovinj, there is direct bus during summer season, timetable you can see below, click on the INFO icons to see operational period of the bus. If you are travelling outside of the season you can travel from Zurich to Rijeka (timetable here) and then continue from Rijeka to Rovinj, timetable you can see here.
If you are going on holiday in Rovinj and has to return to Zurich, you should buy a return ticket as this is cheaper then two single tickets