Stunning limestone karsts, colourful fish and longtail boats
Sea based fun
Ang Thong National Marine Park is a group of 42 pristine islands which have towering limestone karst formations. Ang Thong is located in the Gulf of Thailand and can be reached through group or private tours. There are a range of operators for these tours which can be done by speedboat, slow boat or yacht - some include snorkelling (which isn't the best here) and kayaking.
The main draw of Ang Thong is the stunning scenery when sailing around the National Park. Gorgeous beaches combined with the hilly, tropical forest backdrop are all visited on day trips, along with the famous Emerald Lake, which involves a climb up some very steep steps to reveal a hidden lake inside the island. The water of this lake is a vibrant emerald colour, hence the name.
Koh Tao and Koh Nang Yuan are two islands located to the north of the Samui archipelago and have, by far, the best snorkelling in the area. Day trips on speedboats only are available (it's too far for yachts or slower boats). The day trip makes stops around Koh Tao for snorkelling (with the chance to see black tip reef sharks and turtles) and also stops on Koh Nang Yuan for a reasonable amount of time.
Koh Nang Yuan is a privately owned islands, well it's actually 3 islands connected by a sandbar and has a resort located on it. Snorkelling is found on both sides of the sandbar and there is a moderately difficult hike to a viewpoint which gives you the chance to take those 'postcard' photos!
Koh Taen and Koh Mudson are located a short longtail boat ride away from Taling Ngam on Samui's south-west coast. Day and half-day trips are available on longtails for snorkelling at these islands.