Do I need a transit visa if I will leave airport and travel to the ship with the cruise ships port

I am told by my agent. I need a tourist visa but then read that this is not so. Would you please clarify this for me.
I am a US citizen and will be traveling to Shanghai March 2012 and will be there less than 24hrs.. I am joining a cruise ship that will be leaving the same night of my arrival. Question is: Will I really need an entry visa or any visa into Shanghai when I will be leaving the airport and traveling to the ship with the cruise ships port agent. I will have joining cruise ship documents proving my destination to the ship and the ships itinerary.. If there is anything else I need, I would appreciate information? thank you very much in advance..