I am staying near St Marks Basilica. Need to fly from Marco Polo at 6:30 AM in the morning for an international flight, meaning that I have to be at the airport by 5. How do I get there, that early?
There is a boat "AliLaguna" leaving from St. Marco square and several other places in Venice going to the airport from 3:40 in the morning until 22:25 at night. One way ticket is 12 EUR per person.
There are also private taxi boats you could book but they are rather 120 EUR.
Remember that the vaporetti night boat makes all stops en route, so give yourself time. There are land taxis at Piazzale Roma that are in and out at that hour, but they are infrequent. I have often waited 10 or fifteen minutes for one to show up. Phone one of the several taxi companies the night before-- they speak English- and book one for the time you wish. The trip takes about 20 to 25 minutes and will cost 32 to 35 euros plus tip. Remember that the vaporetti nite boat makes all stops en route, so give yourself time. The boat taxis to the airport need booking as well -- and are MUCH more expensive -- they leave you off at a distance from the terminal, and at that hour the shuttle bus is not running -- you have to walk.
What I have done in the past is have the hotel arrange a water taxi for me. The taxi will be the fastest route and takes you right up to the gates.