Our Viking cruise ends in Venice and it appears Marghera is the port. We want to go into Venice after final disembarkation to stay two nights before returning home. Is there direct transportation by water or ground from there to Venice, or to the airport from where I can easily return into Venice?
Take a taxi to train station in Mestre
Then train to Venezia S Lucia ( about 10 min ride)
You don’t want or need to go to airport
If you want to go to Venice for a couple of days, I'd take the taxi to Piazzale Roma and get a hotel walking distance from there, then you will be close to the bus station for the bus to the airport.
Just take a taxi to P. Roma as Sam suggested. If your flight home leaves in the early a.m., staying near P. Roma will definitely be an advantage as you can easily get a bus or land taxi to the airport.