There was a group of 20 of us on this cruise, ages from late 20's to early 70's.
We arrived in Venice on the overnight train from Paris with LOTS of luggage in tow. As you arrive on the train on the causeway, you will see all of the ships in port so you already have an idea as to where you're actually going.
Some of the group split off, and walked over the 3 bridges with their luggage, and the rest of us bought the 24 hour water-taxi ticket for the short ride down to Piazza Roma for the free shuttle to the ship. Note: YES, you can walk this even with luggage as long as you're in decent shape and don't mind sweating a wee bit!