Do i need a Visa for my cruise?

Hi i am staying in an hotel in Beijing for 3 nights (Arrive 3.15pm 7th September board cruise 11am 10th September depart 6pm) before going on a cruise. Not sure if i need a visa or not as it will be 72 hours before i board but i will not be gong back to Beijing as it ends in Hong Kong. Thanks in advance for your help.