Best way to get to Venice's Marco Polo from San Marco area

How di you mean "best"? Will you be getting vaporetto passes? How are you arriving in Venice? How long will you be in Venice? Need answers to define "best". There are 3 basic ways.
1. Water taxi is fastest and most expensive, about 120 EUR for the group.
2. Alilaguna scheduled boat. 15 EUR per person, 1 hour and 20 minutes from San Marco Giardinetta.
3. Vaporetto plus No 5 bus. About 1 1/2 hours. If you will have an unexpired transport pass, you can add the airport bus when you buy the pass for 6 EUR per person. Otherwise it is 14 EUR for the combined ticket.

Want to make your grand daughter remember the trip forever? Spend the $110 and take a water taxi from your hotel to the airport. It is a really great trip in itself and one of the best boat rides you can find. Give yourself an hour for travel time. Have your hotel get you a taxi, maybe they have better rates. The fee includes all of you and your luggage. I am sure you won't regret it. You will talking about it in the years to come and it is a great way to see the city once last time!

Sam in Green Bay, we are spending 4 nights and 3 full days in Venice. We are staying in an Airbnb. I am inclined to take the vaporetto and bus #5 combo.
Gordon in Portland, your splurge is an enticing idea, but was thinking of splurging on a romantic - albeit with grandma and grandpa - gondola ride the last night in Venice. But, won't rule out the water taxi. It may come down to how tired out we are on our last day in Italy.
I read somewhere that Delta recommends getting to the airport 3 hours before flight time. Do you think this is necessary? We have not flown into or out of Marco Polo, so not familiar with the airport.
Thank you for your advice.

Vicki, how are you arriving in Venice and how will you get to your lodging? You asked about getting from your lodging to the airport, but just wondering how you plan to get there in the first place.

There have been many discussions here about the options for getting to and from the airport. We went with the shared water taxi, which costs a whole lot less than the private water taxi (depending on the number in your party).

Vicki - Marco Polo airport is a small very crowded place and takes forever to get thru security. We have done it twice and both times we were there at least 2.5 hours before flight time. Granted we were making connections in Paris and Amsterdam. Once you are through the check in & security then you can relax with a leisurely coffee etc.

Are you arriving by train? Is your flight home direct to North America?
There is a very nice "Trip Planner" now at ACTV, the Venice Transit Authority.
http://muoversi.venezia.it/en

There is a little quirk in the trip planner. It automatically adjusts your time to Venice time. So for me, in Green Bay, which is 7 hours behind Venice time, I have to input 1:00 am as the arrival time in order for it to show me an arrival at Venice airport of 8:00 am. It is showing 1 hour to get to the airport by taking the No 2 boat (direction Tronchetto) from San Marco "C" dock to Piazzale Roma, then the No 5 bus to the airport. Buy the "Nave + Bus" ticket for 14 EUR.

Venice airport is not that big. Delta (or anybody for that matter) will always tell you to arrive 3 hours before your flight time. As it appears that your flight is direct to Atlanta, there may be some extra security at the gate, So you want to be at the gate 1 hour before the departure time. It should only take less than an hour to get through the check in line, check your bags, go through security, and walk to the gate. I think 2 hours is enough, but that is just my opinion. 2 1/2 hours should be plenty.

Thanks for many replies. In response, to marrowbabette, we are arriving in Venice by train and plan to take the Vaporetto No. 2 to San Marco. Our Airbnb is 50 meters from there, so we'll walk that.
To Shelly, thanks for the heads up on crowds at Marco Polo airport. Remains to be seen what the US airlines decide re: carrying on laptops and tablets. Speaking of that, brings up a big question re: security in putting tablets in checked bags.
Lastly to Sam, I will check out the Trip Planner on ACTV site you provided. What you related is about what we were thinking would work for us. And, we'll try to give ourselves 2.5 hours before flight time. Yes, we are going direct to Atlanta.
Best.
Vicki