Marco Polo airport transfers-

It took about an hour from San Zaccaria to Marco Polo a few years ago when I took the Alilaguna early in the morning. We made one stop at one of the islands, probably Murano. This might be faster for you than the bus and vaporetto but slower than a taxi.

Unfortunately, it looks like only the Blue line goes to San Zaccaria and it is one of the last stops, so it is almost 1 1/2 hours. The Orange and Red lines go to San Marco Giardinetti a bit faster. Look at the schedule here: http://www.alilaguna.it/

I'd take the bus/vaporetto combo. Unlike water taxis and the Alilaguna, you don't have to walk 1/3 of a mile to get from the airport to dock. The bus picks you up at the airport door, takes you to Piazzale Roma where you transfer to the vaporetto. You can take the slow No 1 boat that cruises down the Grand Canal making all the stops, or the 2, 4, or 5 boats that go around to the south and are a bit quicker.

I would also recommend the bus/vaporetto combo. FWIW, the Alilaguna is loud (engine) and packed (people and suitcases). It also sits VERY low in the water so hard to get any decent views. And did I mention the diesel exhaust?

And the dirty windows. . . . I used to like the Alilaguna, but no more.

With the bus to Piazzale Roma, you can board the vaporetto at the beginning of the line and have a chance to get a good spot for the "cruise" down the Grand Canal. I never tire of that.

One other advantage of the bus/vaporetto combo is that, if you're getting a vaporetto pass (and it makes sense for most people, since individual vaporetto rides are very expensive), then that ride is included. And you can add the bus ride for a small surcharge. So, this is by far the cheapest way from the airport to San Marco.