boat from Marco Polo to St Marks's square

There is no express boat. a few early boats make reduced stops, but not that big a deal.
http://www.alilaguna.it/en/linea-blu/blu-timetable

what time do you arrive?

1245 is my scheduled arrival.

The orange line may be a bit quicker and goes on the hour and half hour. Still takes 72 minutes and stops just outside San Marco.

Sounds like you got some great advice. To this I have added an excerpt from the Venice 2014 guidebook regarding the Alilaguna boat:

These boats make the scenic (if slooooow) journey across the lagoon, shuttling passengers between the airport and a number of different stops on the island of Venice (€15, €27 round-trip, €1 surcharge if bought on boat, €1-2 less if bought online, includes 1 suitcase and 1 piece of hand luggage, additional bags-€3 each, roughly 2/hour, 1-1.5-hour trip depending on destination). Alilaguna boats are not part of the ACTV vaporetto system, so they aren’t covered by city transit passes. But they do use the same docks and ticket windows as the regular vaporetti.

The orange line (linea arancio) runs down the Grand Canal, reaching Guglie (handy for Cannaregio hotels, 45 minutes), Rialto (1 hour), and San Marco (1.25 hours). For a full schedule, visit the TI, see the website (www.alilaguna.it), call 041-240-1701, ask your hotelier, or scan the schedules posted at the docks.

thanks for the info. The big question is whether I will be so jet-lagged that I fall asleep on the boat! I guess a double or triple expresso is called for at the airport!