The ferry from Venice to Rovinj is the fastest way of transferring between the cities, the sailing time for the 115 Km (62NM) long route is from 3 – 5 hours, depending on travel date and which of the two ferry companies you sail with. During the main summer season there are up to 12 weekly departures in the shoulder season a few weekly departures. The catamarans depart from Venice around 5pm. As an alternative to the catamaran, it is possible to take a bus from Venice or Venice Mestre to Rovinj, driving time of the bus is 4-5 hours.
When boarding in Venice, please keep in mind that you have to pass through border control and customs, so make sure to be at the port in due time and remember the necessary document. The ferry companies recommend you should be at the port 1 hour before departure, but if you have luggage which needs to be checked-in and have never been to the port before you might want to be there a bit earlier.
TIP: If you intend to return back to Italy, Venezia Lines offer the possibility that you can buy a discounted return trip from different ports, so you could e.g. go from Venice to Rovinj and Return from Porec to Venice, or from Pula to Venice in that way you have less travelling back and forth in Istria.