I am almost 100% opposed to using motoscafi (water taxis) if it can be avoided. Convenience, speed, and privacy aside, this mode of transportation is, in my humble opinion, the biggest rip-off in Venice.
Fares (which must be paid in cash) vary wildly, depending on the bullying skills of the operator who is negotiating your price. I have heard of a motoscafo making the trip to Venice for a flat fee of 100 euros. I have also been present when the charge was 70 euros per person to the first stop, 20 euros additional per person for each stop thereafter. I have heard a fare amount settled on between passenger and driver, and then listened while the driver casually added on a 20% festa (holiday) charge, insisting the fare is always higher on the feast day of St. Somebody or Other. And I myself have agreed on a price of 90 euros, then had the driver demand an additional 10 euros at the end of the ride because he had asked if I wanted to take the Grand Canal route instead of the longer backwater way, and I had unwittingly said, Va bene. Did I pay him this piece of extortion money? He was sitting on my suitcase while he explained this unexpected fare increase to me. I could clearly see any further negotiation attempts would be in vain.
I grant you, there is the convenience of door-to-door delivery to be considered. But remember, if your hotel or other destination does not have a water gate, you will end up walking some distance anyway. (Ask in advance if this is a real concern.)
So, unless you've got money to burn, or you are traveling with a disabled person, or yours is a large group all bound for the same place, resist the temptation to jump into a waiting motoscafo outside the airport. Instead find the Alilaguna kiosk inside and buy tickets there. You will pay only 12 euros per person. Alilaguna boats stop at several convenient locations around Venice, as well as Murano and Lido. And you will enjoy the same water approach to the city as the motoscafo patrons with their lighter wallets.
You can save even more if you purchase 3-euro tickets for the #5 bus. It will take you straight to Piazzale Roma, Venice's bus station, quite quickly, albeit by a far less scenic route. From there all vaporetti (water buses) are available to take you anywhere you need to go.
