What is the easiest way to get from the Venice cruise ship terminal (Marittima) to the train station, either in Venice or Mestre?
It just depends on where you are docked. The last time we were there, we walked. Try to determine the exact pier and then use Goggle Earth to get a view.
After you locate your dock relative to the RR station, determine how far it is and walk to it if possible. The train ride from Venice to Mestre is about 5 minutes.