This really depends on how much you will be using the water transportation system while in Venice. Once in Venice most visitors simply walk to get around and only use the Vaporetto once or twice. You can get a 3-day Vaporetto pass for 40 euro, which gives unlimited use of the Vaporetto and you can add a one-way bus trip from Marco Polo to Piazzale Roma for 6 euro. From Piazzale Roma, the Vaporetto will get you to the Giglio stop.
If you think you will be doing more walking than water transportation then an Alilaguna one-way trip from the airport to the Giglio stop is 15 euro, 14 if purchased online. Taking the land bus followed by the Vaporetto to the Giglio stop is also 14 euro. Each additional individual ride on the Vaporetto without a pass cost 7.5 euro. So you need to consider how many times you might use the Vaporetto, at 7.5 each, before it would be more economical to buy the 3-day pass.
To get to the cruise ship terminal there is the new people mover that goes from Piazzale Roma to the cruise terminal. The cost is just 1.5 euro. If you have a lot of luggage you may want to consider a taxi from Piazzale Roma to the cruise terminal as using the people mover with multiple bags could be a challenge. If you have just one suitcase each the people mover will work fine.