Best way to get from St. Mark's Square Venice to Marco Polo airport?

This website gives you the options for getting to the airport from various locations.

Looks like the water bus (every half hour, takes a little over an hour) might be your best option (vs a half hour walk to the St. Lucia train station).

Since there are several vaporetto stops near San Marco, it would be a good idea to check out which dock you want, confirm schedules there at the dock, and buy tickets (which you activate before boarding) the day before. After the vaporetto, you can catch an airport bus (20 minutes travel time) at Piazzale Roma. If it fits your trip destinations, consider Rick's Mediterranean Cruise Ports guidebook.

That website Lee linked is a little confusing as the "motor boat" and "water bus" icons are reversed. That is, press the motor boat icon and you get the Alilaguna water bus. Press the water bus icon and you get the water taxi ( motoscafi.

Basically there are three ways to the airport:

Water taxi, which is exactly as it sounds, a private taxi. Fastest and mst expensive.

Alilaguna, which is a water bus but different from the vaporetto which are also water buses ( but do not go to the airport). The Alilaguna has limited pickup points and some of the boats are small, but you should have no trouble boarding at San Marco, as that is where the orange Line originates. It will take you down the Grand Canal and out a side canal to meet the lagoon, then across that to the airport. The journey takes an hour and a single ride is 15€ ( or save 1€ by buying in advance online).

The trouble with the Alilaguna is the boats are not all that comfortable and the windows are generally dirty or fogged so you cannot see out. So you don't really see much as you cross the lagoon.

The third way is to take a vaporetto ( regular water bus) down the Grand Canal to Piazzale Roma. This is as far as buses and taxis can drive. You can catch a regular land bus to the airport from there. The vaporetto is €7 for a single ride, and the bus is around €6. There may be a combo ticket but we have never done that so I am not sure.

Another option is a shared water taxi. While private water taxis are expensive, a shared water taxis is an affordable option. I've used the following link to arrange transportation from VCE to the vaporette stop nearest to my hotel (venicelink.com).
They'll make a few intermediate stops to pick up other passengers, but since the space is limited they usually don't make more than two or three stops.
Depending on what time your flight is leaving VCE you may want to consider staying near the airport the night before. There are several nice hotels in the area that cater to early departures, including transportation to the airport.

http://www.italianairportguide.com/vce/venice-airport-busses.htm

Local bus is cheap. But you will need to walk and pull your roller bag past the Piazzale Roma termination point.

Actv Bus Services

Actv’s No 5 bus travels to the Piazzale Roma from Marco Polo Airport.
These are orange buses that operate between the hours of 4am and
midnight. The bus stops are found in front of the Arrivals terminal.
The Actv Bus provides no storage for luggage. This bus service may
take a bit longer than the ATVO buses as there are quite a few stops
along the way, but it is the cheaper of the two services.

Tickets can be purchased at the public transportation booth in the
Arrivals Terminal. Bus tickets must be validated in the stamping
machine next to the bus stops before boarding the bus.

Actv No5 Buses depart for Marco Polo Airport from the A1 stop at the
Piazzale Roma. This stop is opposite the large parking garages nearby
the newsstand.

Actv Buses also travel to Mestre Railway Station. This is bus No15.

Passengers at Marco Polo Airport wanting to travel to Padua can use
the SITA bus service. During peak hours they leave the airport every
half an hour. Travel time is approximately an hour and ten minutes.

Buses from Marco Polo Airport are a very comfortable and economical
travel option, although passengers that need to travel further into
Venice should use the Alilaguna Ferries (Venice’s public boat
service).

Edgar's reply certainly fits this traveler's budget. Yet water taxi would be my more expense option. Get close into Venice whenever you can do it. Remember the smile on George C's face, as cameras rolled during his pre-wedding boat ride into the city.
Experience of the city environment is a trip bonus. Our week RS tour of Venice years back featured a whole day on water by private cruiser. The islands we stopped for visits seemed like our own as we pulled into the dock in an island heart. We got right out on the lagoon with the fisherman, the goods delivery people, and smelled the clean, if murky, waters on every side. A water taxi scheduled early, with instructions to take the scenic route, will cost but be worth every euro cent and become a memory treasure.

"I guess, the best way to get to an airport is to book a taxi from taxi transfer service."

Given the particulars of Venice, using a Taxi is often not the best option. I would also recommend using the Alilaguna boats. These are more of a "bus" than a "taxi", so the costs are more reasonable. If you don't mind paying more, Taxi or private transfer from Piazzale Roma is also an option.

Many smaller European airports, like Marco Polo, have early morning flights, like 6:00 a.m. You'll be flying to a larger gateway airport where you'll sit for a few hours waiting for mid day flights home. It's hard enough traveling home without having to get up at 3:00 a.m. to make such an early morning flight.

I would consider moving out to an airport area hotel with airport transportation to avoid having to get up so early. There are quite a few large and small hotels close to Marco Polo that fit the bill.

When making travel plans, we try fly home from larger cities--rather than taking any 6:00 a.m. flights. From Italy, you're basically looking at Rome and Milan for direct flights to the U.S. at normal times.

Kiwitaxi is a land taxi service ( using cars). They will take passengers for Port of Venice to the airport very expensively (€50 and up for that is €6 bus ride). They cannot pick people up at San Marco as you need a boat for that.

We use the Alilaguna as a good compromise between price and convenience. It is not that great a boat ride, however, as the windows are usually dirty so you can't see out all that well.