I actually just Googled and found my answer.
EMBARKATION PROCEDURE VALID FOR CRUISES FROM VENICE
All Health and Safety and embarkation procedures, including luggage drop-off and meet & greet, will be held in Venice Maritime Station (Venice Passenger Terminal).All embarking Guests will then be transferred to Venice Marghera, where they will board the ship after a quick document check.For safety and operational reasons, it will not be possible to perform a pre-embarkation procedure in Venice Marghera. Guests showing up in Venice Marghera on their own will be asked to go to Venice Maritime Station (Venice Passenger Terminal) for the check-in operations and might risk being late for embarkation.