I live in Venice!!!
There are three possiblity: 1-the best way is to take aquataxi from airport directly to the pier (terrific) you'll arrive under the Marco Polo but it's expansive because it costs about 80$ but aquataxi can take 8 people with the same price.
2- By taxi and it takes 30 mins and it costs 40$
3- By bus, it is cheapest but it takes 45mins and it is not very good with lagguages.
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The easiest way to do it is, of course, to purchase the transfer through your cruise line. The cruise line should have somebody meet you right outside of the airport's customs area and direct you to transportation to the ship.
Failing that, a taxi looks like the best alernative.