I'm having the same issue, but the most cost effective method seems to be by train. A 15-20 minute taxi ride from the port to the train station is supposedly all it takes. Then one must hop on TreinItalia for the 3 hour ride to Venice (San Lucia). As you step out of the San Lucia station, you'll get a near immediate view of the Grand Canal. The San Lucia station is supposedly about a 25 minute walk to Rialto Bridge. Now all this information is based on the 60 minute googling and youtubing I did. If anyone has any updates, please share. I was a looking at a train ride for 6 people which seemed to range from about 50 to 130 Euros. Additionally, the trains seemed to leave every hour which sounds rather convenient.