The carpark is at the "top" of the Grand Canal, not far from the main railway station. It is about 20 minutes by road (bus or cab) from the airport to the carpark. You can hop on a Grand Canal vaporetto (water bus) from the carpark/bus depot.
TedC is right, but please understand that he is referring to water taxis, not water busses. The vaporettos (busses) are Venice's answer to streetcars. You can buy full day or multi-day tickets for reasonable money and ride them all day long if you want.
The best way, as far as price, is to simply take a taxi (land). It should take no more than 20 min. An alternative (much more romantic and fun) is to take a water taxi. These are beautiful wooden speed boats with a covered and open area. The problem is that they are very expensive. From the airport pier to the ship would be about $90 as opposed to a taxi which would be a fraction of that cost.