Snorkelling on the south coast is worth doing, but the honest version of what to expect is different from what most travel blogs describe. The coral reefs on the south coast have suffered significant bleaching over the years and the water visibility varies a lot depending on the season, the weather, and the specific spot. What the south coast does offer reliably is calm, warm, accessible water and some genuinely good turtle encounters if you go to the right place at the right time.
The best time to snorkel on the south coast is between November and April when the sea is calmest and visibility is at its best. From May through October, the southwest monsoon makes the water rough and murky at most spots. Some locations are sheltered enough to snorkel year round, but conditions in the peak season are significantly better.
Polhena Beach, near Matara, is the most reliable snorkelling spot on the south coast for most travellers. A natural coral reef runs parallel to the shore and creates a shallow, protected lagoon that stays calm even when the open sea is rough. Green and hawksbill turtles feed on the seagrass beds and reef regularly, and sightings are consistent enough that most people who come specifically for turtles do see them. You do not need a guide or a boat. You can walk in from the beach and snorkel within 50 to 100 metres of the shore. Snorkel gear can be rented from local vendors on the beach for a small fee. The reef itself is not the most dramatic coral scenery in the world, but the turtles are the real draw here and they are worth the trip. Go on a weekday morning to avoid the weekend crowds of local families.
Unawatuna is the other confirmed snorkelling spot with genuine marine life. Rock Island, a rocky outcrop just offshore in the bay, has coral formations and a reasonable variety of reef fish including parrotfish and butterflyfish. The bay is sheltered, the entry from the beach is easy, and the water is clear in the dry season. The snorkelling is not world-class but it is accessible and enjoyable for anyone from beginners to people who just want an easy hour in the water. Some operators offer short boat trips from the beach to reefs slightly further out if you want more variety.
Weligama bay is calm and shallow enough to snorkel comfortably but the marine life is limited compared to Polhena or Unawatuna. It works as an introduction for complete beginners or for children who want to try snorkelling in a gentle environment. Do not come to Weligama specifically for snorkelling.
Hikkaduwa has a marine sanctuary just offshore with a coral reef accessible by boat. The reef inside the sanctuary is patchy in quality after years of tourist pressure, but hiring a boat to go to the outer reef a few hundred metres further out gives better visibility and more marine life. Most visitors who snorkel at Hikkaduwa do so from a glass-bottom boat, which is a low-effort option for families or people who do not want to swim.
One practical note. If you are snorkelling at any south coast beach and someone on shore offers to guide you to the turtles for a fee, you do not need them at Polhena. The turtles are in the lagoon and you will find them by swimming out calmly and looking. The guided tour markup at well-known turtle spots is real and unnecessary.