If I fly into beijing and visit the great wall and stay 48 hours and then go to hong kong to board a cruise ship, do I need a visa. (all within a 72 hour period)

How do you go to HK from Beijing? If you take flight within 72 hours for HK, you won't be required to apply for one. However, if you plan to take train or other transportation tool (cruise, for example) to HK, you have to apply for one from the local PSB when you are in Beijing.