Thanks to the location and geography for Koh Tao, you can dive all year round. There are 27 dive sites to choose from so plenty of alternatives to find sheltered areas.
During the SE Monsoon season in November-December the visibility is at its worst, ranging from 5-10 meters. It gradually improves in January and from then on you can count on 30+ meters.
The deeper dive sites are spectacular as you will be able to see from the bottom (30 meters) all the way to the surface.