I think that this reply could be too late. However, others could probably benefit as well.
I dived in Green island more than 2 years ago, that was before the "big mushroom" (da xiang gu) fell.
That time, I took train from Tai Zhong to Tai Dong (approx. 6 hours), took a cab from train station to ferry terminal, transfer to ferry terminal. I stayed in the 8bar for lodging. You can rent a scooter to go around the island. The island is not too big. There are a lot of food stalls and hot spring on the island. I would recommend to stay for 3D2N to explore the island and the underwater. Most of the dives are shore dives with rocks and shells on them, thick booties are strongly recommended.