We took a bus into Venice from Marco Polo. There are plenty of representatives inside of Marco Polo that can help you choose the right bus.
The first time we went to Venice we traveled outside of a cruise. There is a website for hotels www.venere.com
that allows you to pick out the cities you wish to visit within the countries. It also shows you maps of the city to include major information such as where train stations are and ports as well as how close the hotels are to points of interest. We stayed at the Principe and took a water taxi for $50 American Dollars last year directly to the port. It was the more expensive way to get to the boat but was well worth the money. The port is really central to everything and by water is only about 20 to 30 minutes from any direction.
Also, fyi..the Principe is within walking distance of the train station. Enjoy your trip. Venice is amazing!