I am using a 3mm today, the water temp = 28C today. You can check my divelog in my website so you can have a brief concept of the water temperature in HK. Normally I just dive from April - Oct in HK.
I go with Oceansky:
http://www.oceanskydiver.com/
I hope you may be able to read those Chinese.
It cost me HK$420 for a Sunday diving excluding gear, but including food and beverage and unlimited tanks.