we would like to travel from Cambodia to Guangzhou by plane on the 11 November, entering China.

We are two German travelers.

And leaving GZ by ferry to Hong Kong International Airport on 13 November, flying to Beijing.
Flight reservation from Hong Kong to Beijing are already made.
Return Flight to Europe from Beijing is on 17 November, already fixed.
Is leaving GZ by ferry/train possible als leaving the country?
Do we need any kind of Visa e.g. 72hours for GZ and 144hours for Beijing?
Thanks for an answer!
Anja