Where do they do snorkeling in Sanya? How much is the price? Can i have the details?

You may go to Wuzhizhou Island to do snorkeling. The price is about CNY 400. The scenery is most beautiful, so you will have a wonderful experience. Have fun!

Shouldn't snorkeling be free? or is this price for equipment rental? Either way it's ridiculous I think. One is better off going to Thailand, The Philippines or Malaysia where everything else (hotel, food,etc) will be cheaper as well.

Well, as I know, if you take the equipment on your own, then the snorkeling is free. They just charge for the equipments and insurance, ans some even include the acitivity items on the water.
The most snorkeling places in Thailand, Philippines or Malaysia also charge for the equipments, altought several places may provide the simple equipments without swim fins.