Arriving at Marco Polo Airport
You can get to Venice from Marco Polo Airport either by land or by sea. It will take over an hour no matter which way you go, as the airport is on the edge of the lagoon, but it is all very efficient and well organised. My vote will always be to catch the waterbus into the centre of the city, it's so great to get onto the water and start to feel like you are really in Venice after all the travelling.
Once you are out of the arrival hall it is a 7 minute level, covered walk to the airport boathouse. The Alilaguna waterbuses all leave from here. You can buy a ticket at the marina or at several other spots inside the terminal. There are three routes from the airport and as you can see, the map is laid out in a similar way to an underground map.
The arancio (orange) and blu (blue) lines are the main routes with the arancia ending up at San Marco and the blu at the cruise terminal. The Alilaguna website has very clear information and route maps.
If you would like to arrive in Venice in one of the beautiful, classic, varnished water taxis (just like George and Amal Clooney on their wedding day) you can buy a ticket at similar spots as the waterbus and also find the boats at the airport boathouse. Much more expensive but...wow.
Alternative land travel to Piazzale Roma is available by taxi or ACTV bus from just outside the arrival hall. Once you are at Piazzale Roma you can either walk or catch a waterbus to your destination.
Alilaguna: The waterbus (vaporetto) service from the airport
Arancia Line: The route to San Marco (orange)
Blu Line: The route to the cruise ship terminal (blue)
ACTV: The bus service from the airport