Many dive shops (outside Thailand) criticize Koh Tao, largely I've found due to competition. Koh Tao is not the most challenging diving, which is why it is a nearly perfect place to teach diving - you get great sealife, in warm water, without strong currents that would discourage many would-be divers. That is why many of the most prolific dive shops (in terms of certifications issued) are on Koh Tao, and some among the highest award winners in quality of instruction as well (there are also bad with the good, so do your research). There is much to photograph in the nearby waters, and many come to experience whale sharks, which seem to be more consistent there than in other places where I have seen them (Honduras, Philippines....).
