Alilaguna from Venice to Venice (city)
A good alternative for an expensive water taxi is an Alilaguna boat. From the cruise port to Piazza San Marco, you pay € 8 per person (one way). A return ticket costs € 15 per person. You can buy the ticket in front of the Stazione Marittima.
TIP: Although they advertise a direct connection between the cruise port and San Marco, this boat makes a number of stops along the Canale della Giudecca, but it gives you the opportunity to shoot some great pictures of Venice.