Taxi to Airport in Venice - how early do they run?

Taxi (the car type with rubber tires) can reach only up to Piazzale Roma. Beyond that, they can't float. Therefore you would need to reach Piazzale Roma first. Taxis run all nght long.
There are however also water taxis (high speed boats with 9 seats). They can run on water and can take you directly to the airport (they also run all night like regular taxi). They are expensive however (about 110 euro from Venice to the airport), therefore they are good for a large party since they hold 9 passengers.
Besides that there are the Alilaguna boats which perform service at regular intervals to the airport (at specific stops). They are sort of 'shuttles'. They are a little cheaper than a water taxi.
In any case reaching the airport from Venice city center (in the middle of the lagoon), is a long trip (over 1 hr with the alilaguna and a little less with the water taxi). Also canals aren't everywhere, therefore you might still neeed to walk a little to the closest dock, if your hotel is not on a canal.
Because of that, I often suggest to depart back to the US from another airport, because reaching the VCE airport in the early morning hours is a pain. Basically you would need to depart from your hotel at least 3 hours prior to flight departure.

See if you can rearrange your trip to arrive in Venice and depart from Rome if you can. (basically travel from north to south)

We DO HAVE a 7:05am flight out of Marco Polo in mid-May, to LAX via Paris.

''Our assorted research'' has shown the M P airport doesn't actually open prior to approx 5:00 to 5:30am, absolute earliest. (This is what we understand... Do further research)
Non-Boat options to Marco Polo:
Trains begin leaving Venice's Santa Lucia train station at approx. 5:05am to Venice Mestre (mainland), where you can taxi (with wheels) to Marco Polo Airport...
Or, switch to bus/take the ATVO bus (with wheels) from Venice Mestre station to Marco Polo.
Or, our chosen option:
ATVO Direct Shuttle Bus (with wheels) begins leaving Piazzale Roma with-in Venice (west side of islands) traveling via over the road/causeway) at 5:00am, and goes direct to Marco Polo, arriving approx. 20 minutes later.
To hopefully accomplish this with ease, we have booked a hotel very near Piazzale Roma, the Best Western Hotel Olimpia..

Trick, as I understand it for such an early departure, is to be very nearby, and to cross as ''few bridges'' (which have some steep steps) as possible, pulling your luggage, which we will do. If you were to stay further east within Venice, you would maybe have to add much walking or water taxi to the mix. I do not know when Vaporetto's (water buses) begin to run. We have yet to find out if we can pre-book our departure ATVO bus tickets. Hope this helps, will follow your questions, hoping to learn more ourselves.

Yes. I'll also add that your 7am flight is a intra Europe flight and you don't need to be at the airport 3 hours ahead. For a within Eu flight I usually get there on line to check in 1 hr to 75 min prior to departure. Venice airport is not that big. So get there by 5:45 and you'll be more than fine.

However, as I mentioned, if you can switch around your itinerary and return from Rome it will be better. Rome airport is 30-35 min taxi ride from the city center. Departing from Florence is good too. The Florence airport is only 15 min taxi ride from the city center.

Just FYI, I am booked on an 11:00 AM flight from Venice direct to Philadelphia on US Air, connecting on to LA. You might look into this.

You could get a room over by the airport with an airport shuttle. There's a Courtyard by Marriott and http://www.anniaparkhotel.it/ that are decently priced.

A number of independent hotels can be seen on GoogleMaps.com if you magnify "Venice Marco Polo" a couple of clicks.

That way, you could be assured that you'd get to the airport on time.

The best way to the airport depends a lot on where you're staying. One Alilaguna Line leaves San Marco around 4 AM, which will get you to the airport in time for a 7 AM flight. The next boat, about an hour later, would be too late. That worked well for me last time, but I was staying very near the stop. Their schedule is posted online.

This is our first trip to Italy and we are taking the Heart of Italy tour adding a few extra days in Venice. We already booked the 6:50 AM flight from Venice (in order to get home in time to pick up our dogs from the kennel…after we land we also have the three-hour drive home from Toronto and have to clear customs at the border...it's always something). Hence, we booked the early flight.

Roberto, you are correct, we connect in Frankfurt so the first leg of the trip is an intra-euro flight...we connect with Air Canada in Germany. And what Lois is saying is what I have been reading - Marco Polo does not open until between 5:00- 5:30 AM. That may give us a little breathing room.

From the posts I've read, I think our best choices, or, at least the simplest are to: 1) stay nearer the airport (thanks, David), or 2) bite the expense bullet and use a water taxi for the quick trip, making reservations the day before, or 3) staying closer to the Piazzale Roma bus station and taking the ATVO to the airport.

We are trying to keep the early departure/airport transportation issue in our mind as we make our hotel reservations. It looks like a package deal to me unless we go the water taxi route.

Dominic, you might look into the B & B Ca dei Polo. I have not stayed there yet, but have a reservation there in May. The reviews are very positive, and indicate that it is a 2 minute walk from both the bus and train stations, very near a vaporetto stop, easily walkable to the major sites, yet in a non-touristy residential area along a small canal. Prices seem very reasonable.