Alilaguna is a separate company that operates public boats to the airport.
Vaporetto and Water Bus are pretty much synonymous.
ACTV is the Venice transit authority that runs the vaporettos (but not the Alilaguna) and the land buses in the city, which includes Mestre and Lido.
If you buy a 72 hour pass and the 6 EUR airport one-way adder, that is good for the No 5 bus that runs from Piazzale Roma to the airport. So from San Marco, you would need to take a vaporetto to Piazzale Roma and change to the bus. They go about every 15 minutes.
The 72 hour pass starts from the first time you scan it on your first usage and continues for 72 hours from that time. Scan your pass every time before getting on the dock or on the bus.
Advantage of the bus is, if you are getting a 72 hour pass, it is only 6 extra EUR to get to the airport if you include it with the pass. It is 8 EUR to buy a separate ticket, and the Alilaguna you pay the whole 15 EUR. Also, the bus drops you right at the airport door. The Alilaguna leaves you at the dock 600 yards for the airport door, although there is now a moving walkway for part of it.
