Look
here for some information about transportaion facilities from the airport to the cruise terminal. The fastesd way is to go by bus to Venice Piazzale Roma. From there you may catch a shuttle bus to your ship or a local taxu (about 5 euros).
The nicest way is to take the ALILAGUNA (about one hour) from the airport to Stazione Marittima.
Look
here for informations about the airport boat and the fairs and click
here for infos about the airport bus
and
here
is the timetable for the Alilaguna boat.
More informations? Please ask.
Regards
Uwe