there are a few places you could try
Phi Phi Island off long beach in fact anywhere on that side of the island. Marks mentioned Phi Phi Isand Villiage before i think good snorkelling off the beach there,off Phra Nang beach walking distance from railaynear Railay beach in Krabi is ok too, otherwise Koh Tao's probably the best ive seen off the beach in fact there i used to be able to just look through the face mask into the water without even getting wet!
Off Haad Yao in Phangan is ok too theres a reef about 100metres off the beach.