Similan Islands Water Temperature by Month: What to Expect

Featured image: Kaewfreedom, CC BY-SA 3.0, via Wikimedia Commons.

Similan Islands Water Temperature: The Short Answer

Expect sea-surface temperatures of about 27–30 °C (81–86 °F) around the Similans. Satellite and archive records for January show a minimum of 27 °C (81 °F), an average of 29 °C (84 °F) and a maximum of 30 °C (86 °F). A yearly average near 27 °C (81 °F) is also quoted for the park. Datasets differ slightly, so treat every figure as an approximate range. Warm water is a constant here. What changes is whether boats can run, how flat the sea is and how far you can see underwater.

Water Temperature by Month

Sourced sea data covers January and the February–April stretch, so the table sticks to those. Cells without a sourced figure say so rather than guessing. Park status reflects the usual mid-October to mid-May season.

January Water Temperature: A Closer Look

January is the best-documented month. Daily readings for 2023 to 2026 sit at 28–30 °C (82–86 °F), mostly 28–29 °C (82–84 °F). Neighboring days rarely differ by more than 1 °C (1.8 °F). A surface-point satellite estimate is not a thermometer at one reef, so a particular dive site can read slightly differently. Air is warmer still, with the same archive listing about 32 °C (90 °F) by day and 22 °C (72 °F) at night.

Does the Temperature Change What You Should Wear?

Rarely, because 26–30 °C water will not chill a swimmer. Snorkelers can usually go in a rash guard or nothing, and our Similan snorkeling guide covers gear in more detail. Divers face a trade-off. Two or three dives on a day trip rarely call for more than a thin suit. A liveaboard of 2–5 days with several dives daily can make a 3 mm suit worth packing for warmth rather than cold. Our Similan diving guide covers dive sites and schedules. Families should think about sun first. Children stay in longer, so a long-sleeve rash guard does more than a wetsuit. Park rules encourage reef-safe sunscreen, so read the national parks authority's rules before packing.

• Snorkelers: rash guard or swimwear.
• Day-trip divers: thin suit or shorty, optional.
• Liveaboard divers: thin full suit for repeated dives.
• Families: sun-protective shirts and reef-safe sunscreen.

Temperature vs Visibility vs Sea State

A thermometer does not predict a good day. Operators report visibility of 15–30 m (49–98 ft) with the best clarity from February to April. That window is not tied to any jump in water temperature, which stays in the same warm band. Sea state is the bigger comfort factor. The WMO sea state code runs from 0 to 9. Code 2 means waves of 0.1–0.5 m, and code 3 means 0.5–1.25 m (1.6–4.1 ft). A forecast page showed waves of about 0.5 m on 12 November 2026, but one day says little about a typical week. Seas can run higher at the start and end of the season. Catamarans take about 2 hours and ride steadier than speedboats, which take 60–90 minutes. Divers choosing between island groups can read our Similan versus Surin comparison. Treat forecast sites as planning aids, not navigation tools.

When the Islands Are Open (and What That Means for Water Temperature)

Mu Ko Similan National Park closes for the southwest monsoon, when heavy rain and rough seas make boat travel unsafe. Operators cited 15 October 2025 to 15 May 2026 for the last season, with closure from 16 May to 14 October. Other sources give slightly different closure windows, so use the park authority's current-season notice from Thailand's national parks department. Our Similan opening date guide tracks the season edges. During closed months, Similan water temperature is only a curiosity. Mainland beaches such as Khao Lak stay swimmable, but you cannot reach the islands.

Getting There and Planning Your Day

Boats leave from Thap Lamu Pier in Phang Nga province. From Phuket hotels, the full journey takes about 3–4 hours each way, including a drive of 1.5–2 hours to the pier. Pickups start around 6:00–7:00 AM. Khao Lak hotels are about 20 minutes from the pier. Our Phuket to Similan route guide compares the options. For schedules and what a full day looks like, see our Similan day trip guide. Visitors must leave the islands before 4 PM, and overnight stays on the islands have not been permitted since 2018. Liveaboards are the only way to sleep near the reefs.

Quick Tips and Mistakes to Avoid

A few habits save the most trouble when planning around the sea.

• Do not rely on a one-day forecast to judge a season.
• Do not assume every month is bookable. Mid-May to mid-October is closed.
• Do not pack a thick wetsuit for 26–30 °C water.
• Do not expect warm water to mean clear water. Visibility follows sea state.
• Pick a catamaran if you get seasick near the season edges.
• Apply reef-safe sunscreen at least 20 minutes before entering the water.