They do tend to move the ships around a bit, we've been moored up by St. Marks Square, as well as near the Station & round in a more industrial area, so you would need to check with your TA or the line.
Something to make sure of is that they ARE docking in Venice, at least one line have started mooring at Trieste which is quite a few miles away to the East, on the Croatian border.
Not much help, but hopefully useful.
Last Cruise: Queen Victoria November 2014
Next: Queen Elizabeth December 2014
Followed by: Minerva April 2015
If your ship is Freedom of the Seas it will definitely not be docked at St Marks Square, it is much too big. The usual place for cruise ships is walking distance to the plaza where all the coaches arrive for Venice and the vaporettas for the Grand Canal leave from there. You should find this information on the Venice Tourist web site.
It is a short walk to the water bus station. John (Cruise Director) will exagerate the distance, as maybe he is reflecting the time it would take for him. It is also not that far to walk to St Marks Square. Maybe 1 mile. Hard to get lost as streets sign poin in the direction to St Marks and Piazza Roma (Bus terminal). [/url]