To catch an early flight from Venice, Italy, consider the Alilaguna water bus, Blue Line. It starts early and has a boat that arrives at the airport at 5:09 am, but it leaves San Marco at 3:50 am. The Orange Line, which is more direct does not start service until 6 am so that won’t work. You can also take a taxi from Piazzale Roma or try a private water taxi. The private water taxi will be expensive but efficient. You will have to consider how to get to Piazzale Roma if you decide on a vehicle taxi. The earliest Vaporetto water bus doesn’t arrive at Piazzale Roma until 5:14 am so that leaves very little time to catch a taxi and get to the airport by 5:30 am.
I think your best options are either a private water taxi or the Alilaguna water bus on the Blue Line if you happen to be staying near the San Marco terminal. The Blue Line goes from San Marco to Lido and out to Murano before heading to the airport. It’s not the most direct route, but if you don’t mind getting up very early, it’s an option. Perhaps you can arrange through your accommodations to have a vehicle taxi waiting for you at Piazzale Roma, and that would give you one more option.