If you have limited time but want to experience crystal clear water in Thailand, doing snorkeling in Similan is your best choice.  You can take a daytrip speedboat to enjoy the paradise within one day time.  The boat takes about 1.5 hrs to reach Similan islands from the main pier. Once you are there, you can enjoy swimming, snorkeling or sun bathing on a nice sandy beach.  Lunch and fruits would be served  on the island where you can sit down and relax yourselve.