Koh Rok and Koh Haa from Ko Lanta

Koh Rok and Koh Haa are the pair of marine park destinations that serious snorkelers look for from Ko Lanta. They lie well to the south and west of the main island, which means a longer boat ride, but the reward is clearer water, more coral and fewer day-trip crowds than the inner islands. Koh Rok is made of two islands, Koh Rok Nok and Koh Rok Nai, joined by a narrow channel and fringed with powdery white sand and a shallow reef that runs close to shore. Koh Haa is a cluster of five rocky islets, with a lagoon that can be swum into and a swim-through cave in the limestone. A full-day trip from Ko Lanta usually visits both, with two or three snorkeling stops and a beach lunch on Koh Rok, run by a long-established local operator that has sold sea trips on the island for years.

The journey is part of the experience and part of the planning. Expect a pickup early in the morning, a short drive to the pier and then a speedboat run of an hour or more each way, depending on the sea. Because it is a marine park, access follows the season: trips typically run from about November to May and stop during the monsoon, when the water is rough.

If the sea is too choppy on the day, the operator may change the stops or cancel, so keep your flexibility. Take a motion sickness tablet before departure if you are prone to seasickness, since the open water crossing can be bumpy.

At Koh Rok, the main attraction is the shallow reef just off the beach. You can walk into the water from the sand and be over coral within a few strokes, which makes it ideal for beginners and children who are confident with a life jacket. Expect schools of reef fish, parrotfish, sea cucumbers and sometimes a blacktip reef shark gliding along the edge.

Visibility is best in the dry season, often from December to March. There is also a small amount of shade, and a simple lunch is served on the island or on the boat. Park rules protect the reef, so use reef-safe sunscreen, do not stand on coral and carry out what you carry in.

Koh Haa, which means five islands in Thai, offers a different kind of snorkeling. The islets are steep limestone, and the water around them drops away quickly, so you see larger fish along the rock walls. The signature feature is the lagoon on one of the islands, a sheltered pool reached by boat or a short swim, and a cave that can be explored when conditions allow.

Currents can be stronger here than at Koh Rok, so guides keep the group together and ask you to wear a life jacket. If you are not confident in deeper water, say so and stay with the guide.

This trip suits travelers who want a full day of quality snorkeling rather than a mix of sights. It is a good second island day after the four islands tour, or the first choice for people who already snorkel. Bring a rash vest, a hat, a towel and a dry bag, and charge your camera or waterproof phone.

The day is long, often nine to ten hours door to door, so plan a gentle evening. Keep in mind that Koh Rok and Koh Haa are also reached from the Krabi mainland piers; the Ko Lanta departure is the closest to the islands and saves hours on the water.