Koh Tao is definitely not doing great compared to 2009 though there is some bounce back lately. I just moved from there to Koh Chang which despite poor visibility still has a lot of absolutely stunning untouched corals though unlike Koh Tao it takes an hour-long boat ride to get out there. Koh Phangan recently made news on massive natural recovery, so there's a lot of hope in the Gulf of Thailand still!