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Via boat it's a two hour trip, via bus it's about 15 minutes. I think it's better if you get off from the train in Mestre and take the bus from there. The buses are in Google maps so you can take the bus to the exact spot you're going to stay. If you want to do the boat trip, it's better if you buy the ticket to the islands tour and you visit Burano, Murano etc.
The bus ticket is 1,5€ and it is valid for 75 minutes.
There is an app for buying tickets (https://play.google.com/store/apps/details?id=net.pluservice.Actv )
you have some options: number 5 "AEROBUS", 15 and 45.
8,00 € - one way ticket to piazzale Roma only lasts for 75 minutes
15,00 € - return ticket to and from piazzale Roma (each is valid for 75 minutes)
14,00 € - AEROBUS + NAVE: one way ticket for the bus to piazzale Roma and for a boat to wherever in Venice - it's valid only 90 minutes so you cannot linger around
you can bring one luggage for each ticket, max cm. 150 for the three dimensions
you can buy all the tickets form this app https://play.google.com/store/apps/details?id=net.pluservice.Actv&hl=it
don't forget to validate the ticket within the app