If you are planning an underwater adventure in the Andaman Sea, the most critical question to answer is: when is the best time to dive Khao Lak? Because Khao Lak is the primary gateway to the world-famous Similan and Surin Islands, diving here is highly seasonal. The marine parks are protected by the Thai government and are only open during specific months to ensure the preservation of the coral reefs and the safety of divers.
Whether you are hunting for perfect 30-meter visibility or hoping to swim alongside a massive Whale Shark at Richelieu Rock, this guide breaks down the Similan Islands diving season month by month.
The Official Similan Islands Diving Season
The Similan Islands Marine National Park and the Surin Islands (home to Richelieu Rock) are officially open from October 15th to May 15th every year.
Outside of these dates, the parks are strictly closed to all tourist traffic due to the southwest monsoon, which brings heavy rains and rough seas. However, within the open season, conditions change dramatically from month to month.
Khao Lak Diving: Quick Reference Calendar
| Month | Sea Conditions | Visibility | Marine Life Highlights |
| Nov – Dec | Calm to Moderate | Excellent (25m – 30m+) | Leopard sharks, Blacktip reef sharks, healthy macro life. |
| Jan – Feb | Very Calm (Peak Season) | Excellent (30m+) | Manta rays begin appearing at Koh Bon and Koh Tachai. |
| Mar – Apr | Calm (Plankton Blooms) | Good (15m – 25m) | Whale Shark Season! High chance of large pelagics. |
| Early May | Variable | Good (15m – 20m) | Fewer crowds, warm water, Manta rays still present. |
| Jun – Oct | Marine Parks Closed | N/A | Local wreck diving only (weather permitting). |
Month-by-Month Breakdown
November to December: The Clear Water Months
As the monsoon winds fade, the dive season kicks off with a bang. November and December offer some of the best visibility of the year, often exceeding 30 meters. The water is slightly cooler (around 27°C to 28°C), making it incredibly refreshing. This is a fantastic time for underwater photographers who want wide-angle shots of the Similans’ famous giant granite boulders without heavy particulate in the water.
January to February: Peak Conditions
January and February are widely considered the absolute best months for weather. The skies are generally cloudless, and the surface of the Andaman Sea is flat and glassy. This makes boat crossings incredibly smooth. During these months, nutrient currents start to shift, bringing Manta Rays to the cleaning stations at Koh Bon.
March to April: “The Big Stuff” Season
If your goal is to see a Whale Shark, this is the best time to dive Khao Lak. As the water warms up (often hitting 29°C to 30°C), plankton blooms occur. While this slightly reduces visibility, it creates an all-you-can-eat buffet for the ocean’s gentle giants. Richelieu Rock and Koh Tachai become magnets for Whale Sharks and Manta Rays during this window.
May: The Quiet End of the Season
The first two weeks of May offer a unique opportunity. The crowds thin out significantly before the parks close on May 15th. The weather can be slightly less predictable as the monsoon transition begins, but the diving remains spectacular, and you will often have famous dive sites largely to yourself.
Water Temperature & Gear Recommendations
The Andaman Sea is famous for its warm, tropical waters.
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Average Temperature: You can expect water temperatures to hover between 27°C and 30°C (80°F to 86°F) throughout the entire open season.
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Wetsuit Recommendation: A 3mm shorty is the most popular choice for diving in Khao Lak. However, if you are doing a multi-day liveaboard and diving up to four times a day, a 3mm full wetsuit is recommended to protect against minor jellyfish stings and keep your core warm during repetitive dives.
Book Your Seasonal Dive with King Andaman Scuba
Timing your trip correctly is the first step; choosing the right team is the second. At King Andaman Scuba Diving Khao Lak, our local experts know exactly how to read the seasonal currents and weather patterns to get you to the best sites at the perfect time.
Whether you are looking for a crystal-clear December day trip or a multi-day liveaboard adventure in pursuit of April’s Whale Sharks aboard the M/V Maxwell, we have you covered.
