If you are in Venice and plan to travel to Porec in Croatia, the fast ferry from Venice to Porec is the easiest and fastest way of getting there, the ferry route is operated from May to the beginning of October, there is departure almost everyday in main season, and a few times a week in the shoulder season. The travel distance is about 108 Km (58NM) and the travel time is about 3 hours.
The catamaran route is operated by Venezialines and Adriatic lines, which both use catamarans on the route, the boats do not have outside seating due to the high speed.
As travelling from Italy to Croatia is leaving the Schengen zone you will have to pass passport control and customs before you board the boat in Venice, so remember to arrive at the port in due time. Normally 1 hour before departure is enough, but on busy travel dates, you might want to arrive earlier.
Alternative to taking the catamaran, it is possible to travel by bus from Venice to Porec, the buses depart from Venice Tronchetto and some also from Venice Mestre.