My suggestion: Stay away from water taxis. They cost a heck of a lot and there is inexpensive and convenient public transportation from and to anywhere.

Anyone who has never been in Venice will probably want to take a gondola. These are even more expensive than taxis but are a must -- if just for the experience. Gondolas are not normally taken as actual transportation, but just for the ride. You can usually get about 4 people in one for the same price.