US citizen visa requirement for embarkation

My husband and I are US citizens holding US passports. We are flying from Los Angeles to Shanghai to embark on the cruise one day in advance. We shall have boarding tickets for the cruise ship. From Shanghai we are traveling to countries that do not require visas including Hong Kong. Do we need to apply for a visa in advance for 72 hours stay in Shanghai before boarding the cruise? Are we allowed the 72-hour visa free entry in order to embark on the cruise? Thank you.