Assuming your arriving the day of the cruise and don't have cruise line transfers, the best way to get from the airport to the ship is to take the bus from the airport to the Piazzale Roma, about 4 euro per person. The Piazzale Roma is the bus station in Venice. If the cruise line is running a shuttle from the Piazzale Roma to the ship, grab the shuttle and you're there. If not, there are two options -- walk or grab a cab. The cab costs about 12 euro. The bus runs every 30 minutes and takes about 20 minutes to get to the Piazzale Roma. Walking should take you another 10 - 15 minutes and a cab less than 5 minutes.
You could take a water taxi also. That will run about 90 euro for up to four people.
Don't take the water bus (Alilaguna) from the airport to Venice. It doesn't take you anywhere near where you'll need to go to get the ship.