for which port do i take ferry from hong kong airport to reach guangzhou?

You can take ferry at Skypier in HKIA. There are ferries that can take you to Guangzhou Nansha Ferry Terminal directly. The only schedule is at 11:15. Service available on every Thursday and Saturday.

Or you can change the transportation. There are also many long-distance buses at the airport that can take you to Guangzhou directly. The ticket counters are located at Counter C10-C13,Coach Station,Terminal 2. And the pick-up station is located at Coach Station,Terminal 2. They are available every day.