I went on the travel tourism site. Listed below. You can use them to get from the Ravenna train station to the Port...
https://www.turismo.ra.it/en/useful-services/shuttle-bus-ravenna-porto-corsini-cruise-terminal/
Train from Venice to Ravenna
Trenitalia has an app for the train times. Make sure to change the time to about 5am, so you can see the full list of time. If not, it will default to your current time and you will miss seeing the earlier times
Booking early is better to save money.
Better to get to Ravenna early then playing it close with getting on the port.