I have a Chinese passport and an American Visa, do I need a Hong Kong entry permit?

But I live in Shenzhen. I have a flight from Hong Kong airport. I understand that i can get the ferry from Shekou to Kong Kong airport and dont need a Hong Kong entry permit to depart from the airport. i have an American Visa and a flight ticket from Hong Kong to America so can i go to Hong Kong a couple of days before my flight and look around. I have been told its possible to do this. If so how? Thank you very much