As someone who dives in Hong Kong waters frequently, I wouldn't waste time diving here, especially when you have so many world class diving spots nearby in neighboring countries. There are a lot of good flight connections from HK to elsewhere in Asia such as the Philippines. How much time would you have for your diving portion? If you can muster up 3-4 days, I would recommend flying out to Manila and go to Puerto Galera or Anilao. Flight is 1 1/2hr, plus a 2 1/2hr drive (+1hr boat ride to Puerto Galera). With more days, you could also head down to Cebu and try some of the great diving around the area like Bohol, Cabilao Island, or Malapascua Island. If you have 5 + days to kill, you could even fly direct to Kota Kinabalu to head to Sipidan/ Layang Layang. There are even direct flights to Bali. Manado/ Lembeh Straits could be doable but you would have to have an overnight stop in Singapore