Exit or transit visa requirement

I am a US citizen on a research vessel scheduled to arrive in Shanghai on April 10th. I do not have a Chinese visa yet, and my intention is upon the vessel's arrival to go straight to the airport and fly back to the US. Do I need a transit or exit visa to do this, and if so, is it possible to get this either before arrival (electronically) or upon arrival? The vessel with be remaining in China for several months, so I don't really have much of an option to depart on the vessel later.