Hi Centrals,
What Paul said above is absolutely correct.
Water temperature was about 25-26 C. I dived the whole week in my old, worn and torn 3 mm wetsuit with a 3 mm vest and a hood. This is not to say that I felt warm and balmy after the second or third dive of a day, but it's definitely doable. Bring a wind breaker or old raincoat though, as getting in and out of the water can be chilly. The wind on Green Island can - at times - be quite strong in January/February.
Also, bring multiple kinds of signaling devices (big SMB, mirror, torch, etc.) as currents are so strong that you easily could end up a couple of hundred meters from the dive boat. Add to that 1-2 m waves and swells and you can imagine that it's easy for a captain to overlook a diver!
To get an impression of the island and what it's like to dive there, go to my photography website by clicking on any of the pics below. The first 3 images are linked to a gallery with some 200+ images of our week-long dive trip to Green Island this February. The last image links to another gallery from previous dive trips to Green Island, with a few more topside shots.
The Hammerheads
The dive site, aptly named the
'Boiling Water Point' in Chinese!
The shark dive site is located about 500 m - 800 m
out from the promontory sticking out in the far back.
Hope that helped.
Thomas