She is a 'ship'. From St Marks you need to catch the vaporetto up to Piazzale Roma. From there you [a] walk, [b] wait for the free bus that sometimes does not turn up or [c] catch a cab. When you disembark the only place in Venice to leave luggage is at the train station - Santa Lucia. So again you have to get back to Piazzale Roma, and one stop from there to the train station.
There is a left luggage store at Piazzale Roma near the bus office, and you can leave your luggage there, I think there is a water bus link from St Marks directly to the port, but there is also the complimentary shuttle bus from Piazzale Roma, you may be told it will be available at the end of your cruise, but don,t relie on this as it run by the port authority and will be carrying passengers from any other cruise boats docked in port that day and consequently very busy. Have a great time, we did this trip last year