Hi acbat,
you can take the Alilaguna Airport boat. It starts at Stazione Marittima and costs € 13 per person.
Look here for the timetable:
http://www.alilaguna.com/
The Venice airport bus is cheaper: only three Euros. Look here:
http://europeforvisitors.com/venice/...port_buses.htm
For further information about the cruise terminal, click here:
http://europeforvisitors.com/venice/...e-terminal.htm
Last but not least look at the port map:
http://www.vtp.it/pages/navi/mappaCrociere.jsp
You will find there a mark for Alilaguna.
Regards
Uwe
If you need more infos, please ask.