You never gave the date that your sister is leaving, but I can answer the question about the ship to San Marco;
I just returned from the Millenium 8/6 cruise.

The ship offers shuttle service from the dock down the Grand Canal to San Marco for $20 roundtrip. I think it took about twenty minutes each way. The shuttles run about every half hour.

A second option is to walk to Piazza Roma (about a ten minute walk) and get the vaporetto.

Someone suggested that your sister sign on for the cruise critic gathering. It's a good idea but we were really disappointed. There were about 62 people registered for ours but I think less than 20 showed up -- probably because it was held at 10:00 on the day after we left Venice and many were tired from the flight and trying to see as much as possible of Venice.

I hope she has a wonderful time.

Luisah: thank you so much for responding to the last remaining question - how to get to Piazza San Marco. Phew! I will pass this info along to my sister. Fodorites are the greatest

You're very welcome. I always like to know as much as possible in advance, so I understand. She can buy the pass right at the shuttle.

Tell your sister not to miss the sailaway from Venice. We went to the very top of the ship (the elevator to the 11th, then up some stairs to the top deck)and got some great photos as we sailed down the canal.

Other photo ops are the approach to Dubrovnik and passing through the Straits of Sicily and seeing the sun set over the cities on the coast.

Luisah: again, thank you so much for more info on my sis's Med cruise. I have sent her your info. She was so pleased to get the info on transport from ship to San Marcos. You're the greatest