Am I exempted from the air passenger departure tax if I travel to HKIA via ferry?

Yes, depending on your ticketing arrangement.

Passengers travelling to HKIA with a Direct Air+Sea booking are exempted. If your flight and ferry are booked together under a single itinerary with a codeshare arrangement, the departure tax will be automatically waived. No further action is required.

If your ferry ticket is purchased separately from your flight, you are still eligible for exemption. However, you will need to manually apply for a refund of the departure tax on the Airport Authority’s official page .