How to get to Marco Polo airport from Venice for 6 am flight

Perhaps transfer to an airport hotel the evening before you leave.

Or, there is always a night vaporetto running in Venice. From P. Roma you could get a taxi, perhaps prearranged the night before by the hotel.

Private water taxi, official site
https://www.motoscafivenezia.com/en/

I believe the first bus from P Roma is at 4:20 which would get you to VCE by 5. That’s plenty of time for that 6:25 flight. The flight will be going to somewhere in Europe and the airlines check in desk probably doesn’t open til closer to 5:30

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hey hey prokochristopher
agree with ChristineH about the private water taxi. could be about 120E depending on where pick-up but so so worth it. we had that issue so just pit money in "piggy bank" for it and added to expenses. this happened to use and we just emailed to reserve it. your hotel will charge you more. we have used a private water taxi 4 times and would do it again.
venicelink.com
is a shared private water taxi shuttle also.
when is this trip taking place and how many people as in "we"?
do what you have to, make that flight from venice. you will have a stop in another big city and then on to USA if that's where you are headed. good luck
aloha

Caveat, it's been a few years, but we found the Best Western Titian near the airport to be a good solution to this problem. Very close, shuttle service early, and while I don't recall the name, there was a restaurant within walking distance that was as good an anything we had found in central Venice.

Thanks for all of your replies. Think we will take the private water taxi as we like to make certain we will get there on time. Love the ability get such great suggestions from experienced travelers!