The best of the snorkeling on Koh Phangan is found on the north-west of the island’s coastline where there are several small reefs to be found offshore. Check out the Koh Phangan map and look out for the snorkeling icon 
which gives you an indication of the best places to don your snorkel and check out the marine life.
The best place to go snorkeling on Koh Phangan is around Koh Ma, a small island that is joined to Mae Haad beach by a sandbar that is often walkable at low tide. You can get a longtail boat from Mae Haad to take you out and hover around while you snorkel there but be sure to check the local knowledge regarding any currents or tidal issues to beware of. Snorkels and masks can be rented from many bungalow operations or you can buy your own from the shops and stalls at Thong Sala, the main town of Koh Phangan.
Apart from Koh Ma there is coral to see off of several other beaches on the island. The most popular snorkeling spots are Haad Salad, Haad Yao and Chao Phao on the west coast whilst Haad Khom on the northern coast is another favourite.
A boat trip to nearby Koh Tao will also serve you well for snorkeling. It’s a diving mecca at Tao but there is also great snorkeling just off the beach at Koh Nang Yuan and Ao Mumuang. The busy Hat Sairee also has some coral close to shore if you fancy have a snorkel off the beach there.