Not knowing your ultimate destination, I looked up flights on Skyscanner between VCE and Frankfurt, Amsterdam, Paris, and London for a random summer day, and found only early morning departures, which you couldn't expect to make after disembarking, and mid-afternoon departures, which put you at those hubs so late in the day that you'd probably have to spend the night there before flying home. So, as you apparently have already figured out, the smartest plan is to stay in Venice that day and spend the night before a morning flight. You asked about the quickest way to the airport, which is probably water taxi at great expense. My suggestion would be to spend the night at a hotel close enough to P. Roma that you can roll right onto an early bus. Any hotel that meets your comfort standards and budget would do, just look for proximity to that bus.
I'm sure you also know that Venice is worth much more than a single day's sightseeing. If you can manage several days there, you can look for lodgings in more appealing parts of the city and spend just the final night near P. Roma. Or you might choose a hotel near the airport. I think there's a Marriott, a Best Western, and the Parkhotel Annia where I stayed after a late-night arrival. But I wouldn't stay near the airport if you have just one night, much better to experience Venice late in the day when your fellow-cruisers are long gone.