The Similan and Surin Islands are two marine national parks in the Andaman Sea, and Khao Lak is the nearest mainland base for both. They are among the most famous snorkeling and diving destinations in Thailand, with clear water, healthy reefs and beaches that feel remote. Visiting is a major highlight of any Khao Lak trip, and a little planning goes a long way toward choosing the right trip, the right boat and the right time.
Similan or Surin?
The Similan Islands are the star: granite boulders, white sand bays, steep underwater drop-offs and big visibility. They suit confident snorkelers and anyone who wants dramatic scenery. The Surin Islands lie farther north near the Myanmar border and feel wilder and quieter, with shallow coral gardens that are easy for beginners and a community of sea nomads called the Moken. If you have time for one, most people choose the Similans; if you have time for two, pick both and enjoy the contrast. Children and nervous swimmers often prefer Surin's calm, shallow bays.
When the parks are open
Both parks typically close for the monsoon, from around mid-May to mid-November, because the open-sea crossing is unsafe. The best months are December to April, with calm seas and the clearest water. At the edges of the season, boats run only if conditions allow, and cancellations are common. Always read the refund terms and keep a spare day in your schedule. If you are visiting in the monsoon, choose Phang Nga Bay, Khao Sok or cultural tours instead.
Boat types
Speedboats are quick, so you spend more time at the islands and less on the crossing. They can be bumpy, with a loud engine and spray, and they usually carry fewer passengers. Catamarans and larger boats are steadier and have shade, seating and sometimes a sun deck, which suits families and anyone prone to seasickness. Premium or yacht-style trips carry fewer people and offer extra comfort. Smaller groups are more flexible, and large groups can feel crowded at the popular snorkel spots.
What is usually included
Check hotel pickup, the national park fee, snorkel gear, a life jacket, lunch and drinking water. Some operators include fruit and soft drinks, and some offer a guide in the water. A few trips charge the park fee separately on the day, which can surprise visitors. Ask how many stops are planned, how long you will spend in the water and where lunch is served. A trip that visits two or three sites and a beach is typical.
Diving and overnight options
Certified divers can add a two-day scuba trip that stays near the islands, giving more dives and calmer early mornings. A discover scuba option is sometimes offered for beginners at easier sites. Overnight trips are simple, with basic accommodation, but they give quiet evenings and the best light. Ask about group size, the dive guide ratio and nitrox, and bring your certification card.
Snorkeling well
Practice with the mask in shallow water, breathe slowly and relax your legs. Keep your fins away from the reef, and never stand on coral. Use reef-safe sunscreen or a rash vest, since the sun is strong and chemical sunscreens harm marine life. Do not feed fish, collect shells or take anything from the park. Watch for currents near drop-offs and stay with your guide. If you are nervous, tell the crew early; they are used to helping.
What to pack
A hat, a light long-sleeved shirt, a towel, a dry bag, motion sickness tablets, water, reef-safe sunscreen and small bills for tips. A waterproof phone pouch or an action camera adds fun. A change of clothes for the ride back is useful, since boats can be cold in the afternoon breeze. Eat a light breakfast and avoid alcohol the night before.
Making it count
The islands are a protected resource, and visitor behavior makes a difference. Choose operators that respect park rules, carry out your rubbish and follow guide instructions. The reward is memorable: turtles gliding past, fish shimmering in schools and a quiet beach surrounded by water so clear it looks like glass. Book early in the high season, and allow a rest day after your boat trip.
A last word on booking
Compare two or three operators on boat size, inclusions and recent traveler feedback rather than price alone, and book flexible dates where possible. Early-season and late-season departures can be excellent value, but be ready for changes. Tell the operator about swimming ability, medical conditions and dietary needs, and arrive at the pickup point on time, since boats leave on a fixed tide and weather window.