Carol, we have seen this questions dozens and dozens of times. In general, the airport check-in counters open about 5:00am. If you get there at 6:00am, you are easily fine for check-in. It's an inter-EU flight which makes it easier. As for how to get there early enough, you have several options. 1) There are two bus lines that depart the Piazalle Roma for the airport. The ACTV bus No 5 departs at 4:35am and the ATVO departs at 5:00am. See www.actv.it or www.atvo.it to make sure of the schedules. Trip to the airport takes about 25 minutes. About 7Euro. The next ACTV and ATVO buses both depart at 5:40am.
2) The fun part. Regular vaporetto service begins at about 5:00am. According to the ACTV schedule that I am looking at, the first water bus (vaporetto) is scheduled at Ca'd'Oro at 5:01. You arrive at Piazzale Roma at 5:14. That's early enough to make either of the 5:40 airport buses. You are fortunate here. Many travelers have flights at 6:00am and 6:30am.
3) The water taxi. You can arrange with your hotel to have a water taxi waiting for you at your hotel. Fare is usually about 90Euro-110Euro for up to 8 people with luggage. It's a private boat.
4) Spending your last night near the airport. Not advisable for this situation. Courtyard by Marriott and Titian Inn are two nice hotels that are in very close proximity to the airport and have early morning shuttles. But, you miss your last night in Venice for this option.
5) Sleeping in the airport. People do this. From what I understand, the lock you in from about midnight to 4:30am or so. You can read more about this at www.sleepinginairports.com