Whole year around it is possible to travel from Venice to Rovinj, the 270 km long trip takes about 4.30 minutes with bus. Compared to other means of transportation, bus is by far the cheapest way to travel for a single person.
Depending on where you are in the Venice area, if might be easier for you to board the bus at the Mestre station. In Rovinj the bus stops at the main bus station, which is with in walking distance of the old town.
Here you can see timetable for bus between Venice and Rovinj.