From the Piazza San Marco:
Alilaguna runs a BLU Line express boat service between the Piazza San Marco and Marittima about once an hour during the daytime. The trip takes 20 minutes.Alternatively, you can take a water bus (vaporetto) to the Piazzale Roma and proceed from there by taxi ( about 5 Euro), (shuttle) bus, or on foot if you aren't traveling with too much luggage. The No. 82 express boat is quickest; the No. 1 vaporetto down the Grand Canal is handy if you've been staying near one of the intermediate stops.
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A one way-fare with the vaporettos (water bus) costs Euro 5,--, a one day Travel card Euro 12,--. Here
you will find the fare page.