in Venice, it is very simple to arrange your own excursions. Here is a link to the cruise port:
You can click on TRANSPORTATION LINKS to get some info . It is not far to Piazzale Roma, from where the so called Vaporettos (public warter busses) start up und down the Canal Grande (Line 1). You can go directly to the Marcus Place.
There is a line too from the green Number 103 in the map to San Marco (Alilaguna). If you want any other links from Venice, please ask.
Have some nice hours in a wonderful city