When you get off the ship, it's just a few minutes walk out to a range of transport options:
This service is organised by the cruise line and cost $25 pp per day for as many trips as you like. We bought tickets from the shore excursion desk but you can also buy them ashore. Best to check before getting off though. It's a large motor boat.
It's the blue and white boat you can see in front of the red building.
Venetian public transport - the 'vaporetto'
You will see a boat stop where you can take this service. Not sure if you can buy tickets on board but would assume so and would be very economical.
City Sightseeing Hop on hop off boat
Yes that's right, there is a Venice equivalent of the big red bus we take in other cities...except it's a boat. It also stops at the port so this would be a great way to get to all the main attractions.