Sorry, there is nowhere to go snorkelling in Cha am or Hua Hin. The nearest place in this area is a small Island called Koh Talu which is a short boat ride from Bang Saphan Noi about 200 kms south of Hua Hin. You can arrange day tours or overnight tours from this area or stay at the resort on the Island.
Other areas you may consider for snorkelling holiday in Thailand would be Koh Tao which is in the Gulf of Siam ( reached by boat from Chumphon - 300 kms south of Hua Hin )and is famous for it's diving.
On the west coast of Thailand have a look at the Similan Islands or Koh Phi Phi.
