Snorkeling can be done all year round in Phuket, but the best time of year is during the "High Season" from November to April, when the sea's at its most placid. Although it may be suitable for snorkeling throughout the May to October "Low Season", remember to take care as there can be an especially strong undertow that can become dangerous along the west coast beaches. Warm water temperatures of around 28°C mean that snorkelers don't need to wear wetsuits, but you are advised to wear a t–shirt to prevent sunburn.
Many beaches have stalls to rent snorkeling gear from for the day, but it may not be of great quality, so it is a good option to consider buying inexpensive masks, snorkels and even fins for yourself. Alternatively if you decide to join a boat tour or charter a boat of your own to snorkel in one of the beautiful snorkeling destination off the Island, then the quality of the gear is better, and included in the rental price.
Many of Phuket's beaches are too sandy to be good for snorkeling, as a good location needs rocks and corals to attract plenty of sea life, but you can try snorkeling at pretty much any beach, just some are obviously going to be better than others. The tsunami inflicted a lot of damage to Phuket's corals and you will see a lot of flattened dead coral at most of the snorkeling sites, but a great deal of corals survived and new corals are growing all the time, so there is still plenty to see.

AO SANE (Jungle Beach):
This beach can only be reached by boat, or by following the road that runs through the car park of Le Royal Phuket Yacht Club in Nai Harn, a few minutes from Sapphire Villas. Sometimes called "Jungle Beach", it is generally quieter and more peaceful than others, because fewer people know of its existence, but it is worth a visit as it has good coral outcrops off its mostly rocky shore.

Found between Nai Harn and Promthep Cape, this beach is considered to have fairly good snorkeling available. Ya Nui's popularity has increased in recent years as it is now a launch point for divers and is lined with beach chairs for rent, but it is still a relatively relaxed, and with it's small, informal restaurant and friendly staff, it's is a great venue for a sun downer as the sunsets.

Most of Kata's length is soft sand, and can have large waves, but the north end of the bay is rocky with clear water and lots of coral, so it a good option about 10 minutes from Sapphire Villas.