There was some ok snorkelling off long beach in Phi PHi, but the real good stuff was 10-15 mins away by long tail boat. Koh Tao has reasonable snorkelling off the beach at some of the back beaches. Both of these destinations will have limited appeal for kids of that age. If you go to AO Nang, you can access some great spots via longtail and you can keep the kids amused with mini golf and snake shows. Krabi Town has shopping centres with games/karaoke/bowling etc and you can take them on some really cool daytrips to fossil beach or the hot springs. Most of the major places like Phuket or Samui really require a day-trip or a fairly rushed half day to access decent snorkelling.

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