The tropical waters of the Andaman Sea are ideal for snorkeling - clear, calm, balmy and bursting with brilliant marine life. Snorkeling can be done year-round in Phuket, but the best time of year is November-April, when the sea's at its most placid. Take care through the May-October low season, when the undertow can become dangerously strong along the west coast. The warm water temperature (27-28°C) means that snorkellers don't need to wear wetsuits, though wearing a t-shirt is advised to prevent sunburn.
At some beaches it is possible to rent snorkeling gear for the day, though it may not be of great quality. Many shops around the island sell a range of masks, snorkels and fins.