Maybe consider "Similan island" as well. My husband says "Similan Island is the best place for scuba dive" but to go there you will need to fly to Phuket and maybe take day trip at Similan Island. Or you can stay at Khaolak then take a boat to Similan. Khaolak is very quiet place tho...afraid you and your parents will get bored there.
But you can also consider to stay in some quiet area in Phuket like Bangtao beach or Naiharn beach and take a day trip to Similan (about 1.30 hrs by bus).
Hope this help ð
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