Just to clarify I am only asking about coming from the US to Venice directly
Then you will have to wait in an immigration line and have your passport stamped, officially entering the European Schengen Zone. By the time that is done, your luggage will probably be on the luggage carousel, then you can walk out the "Nothing to Declare" door. It is that waiting in the immigration line that is an unknown. Everybody on the plane will have to do that, but most European citizens will go through a separate resident's line, which is often quicker. You have to go through the "Other Passport' line. If another airplane from outside Shengen lands just before you, that could be a big line.
Just let the taxi company know you are arriving from the US and the flight number. Once you enter the airport with your luggage, follow the signs to the boat dock. You will go upstairs (or elevator) and take a moving walkway on the land bridge to the dock, then downstairs and outside to the dock.