Similan Islands Conditions Guide: Seasons, Visibility, and Currents
The Similan Islands operate on a schedule that no other parameter matters more than: the park is open from mid-October to mid-May and closed the rest of the year. Within that seven-month window, conditions vary dramatically — from 15-meter visibility and irregular seas in October to 40-meter clarity and flat-calm water in February. If you're planning a trip, the month you choose determines what you'll see, how comfortable you'll be, and which sites you can dive.
This guide breaks down every month of the season with hard numbers so you can time your trip with precision.
The Numbers That Matter Most
Water temperature: 27–30°C (81–86°F) year-round across the open season. A 3 mm shorty or thin full wetsuit is sufficient for most divers. The warmest water (29–30°C) comes in April and early May at the tail end of the season. The coolest (27–28°C) runs from October through January.
Visibility: 15–40 m (50–130 ft). The best visibility runs January through April when the northeast monsoon has settled in and the seas are at their calmest. During this period, 25–40 m is the norm. Visibility drops to 15–25 m in October and November as the season opens and monsoon transition stirs the water column.
Current: This is the most variable and important factor. The eastern side of the islands is almost always mild — this is where beginners dive. The western side, open to the Andaman Sea, can run moderate to strong (1–3 knots) at any time, especially at exposed pinnacles (Elephant Head Rock, Koh Bon Ridge, Koh Tachai). Drift diving technique is required on western sites. The northern pinnacles (Koh Bon, Koh Tachai) and Richelieu Rock are always current-sensitive — dive guides plan tides carefully.
Depth range: 2–40+ m. Shore-accessible sites hit only 2–14 m. The best reef and wall diving sits at 12–30 m. Deep pinnacles and seamounts drop to 35–40 m.
The Park Closure: Mid-May to Mid-October
The southwest monsoon brings heavy rain, rough seas, and dangerous conditions from mid-May through mid-October. During this period, Mu Ko Similan National Park closes entirely — no diving, no snorkeling, no tourism of any kind. This annual closure is critical for reef recovery and has directly contributed to the Similans' ability to maintain world-class diving quality year after year.
If you're planning a trip, note that the park typically reopens around October 15 and closes around May 15 each year. Exact dates vary slightly and are announced by the Department of National Parks. Some operators run the first trips as early as the second week of October if conditions permit.
Month by Month
October (Mid-October – Late October): Season Opening
The park reopens mid-month and conditions are transitional. The southwest monsoon is fading but residual swell keeps things lumpy. Water temperature sits at 27–28°C, visibility ranges 15–25 m (improving through the month), and currents are irregular.
Who should go: Budget-conscious divers and early-bird planners who want uncrowded boats. October often has the lowest prices of the season, and liveaboards run with more space per diver. The trade-off is unreliable conditions — you may get flat-calm days or 1.5-meter swell.
Marine life: Early-season manta rays start appearing at Koh Bon. Reef fish are abundant. Green turtles and leopard sharks are reliably sighted on eastern sites.
Gear tip: Pack a 3 mm full suit (you'll be comfortable) and a good surface hood for the boat ride — the wind during October can make the journey to the park chilly.
November: Improving Conditions
November is a transition month but trending good. The northeast monsoon begins to establish, winds moderate, and seas calm significantly compared to October. Visibility climbs to 20–30 m, water temperature holds at 27–28°C, and currents settle into more predictable patterns.
Who should go: Divers who want to beat the peak-season crowds but still get good conditions. November strikes a strong balance between price (still below-peak) and quality (visibility is already good).
Marine life: Manta ray activity at Koh Bon picks up. Leopard sharks are common on sandy patches. Whale shark sightings begin, though they're still infrequent this month. Reef life is in full swing.
Gear tip: Same as October — 3 mm full suit. Bring a dive light as the shorter days mean afternoon dives can be darker.
December: Peak Season Begins
December marks the start of true peak conditions. The northeast monsoon is fully established, seas are calm, and visibility reaches 25–35 m. Water temperature sits at 27–28°C. Currents are generally mild to moderate on the eastern side and moderate on the western boulder sites.
Who should go: Everyone. December is the first month where conditions are consistently excellent. It's also when Christmas and New Year holidays make liveaboard spots scarce — book 3–4 months ahead.
Marine life: Leopard sharks are at their peak visibility on sandy patches. Manta rays are reliable at Koh Bon. Grey reef sharks patrol the western walls. Eagle rays and turtles are common. Whale sharks become more frequent at Richelieu Rock.
Gear tip: 3 mm full suit. December is one of the coolest months of the season — the boat ride in the morning benefits from a windbreaker.
January: Peak Conditions
January offers the best combination of conditions in the season. Visibility hits 25–40 m — the year's peak — and the seas are as calm as they get. Water temperature is 27–28°C, the coolest of the open season but still very comfortable in a 3 mm suit. Currents are mild to moderate, with predictable drift patterns.
Who should go: Divers who prioritize crystal-clear water and calm seas above all else. January is the top month for underwater photography, with perfect lighting conditions and maximum visibility.
Marine life: Mantas at Koh Bon are at their peak. Grey reef sharks are abundant. Leopard sharks are everywhere on sandy patches. Eagle rays cruise the walls. Whale sharks at Richelieu Rock become more frequent. Schools of fish are at their densest.
Gear tip: 3 mm full suit. A dive computer and SMB are essential — drift dives are the norm on western sites.
February: Whale Shark Month
February delivers the same excellent conditions as January — visibility 25–40 m, water 27–28°C, calm seas — but adds one critical ingredient: this is peak whale shark season at Richelieu Rock. Sightings are reported almost weekly, and on good days, multiple whale sharks are encountered on a single dive.
Who should go: Anyone who has "swim with a whale shark" on their diving bucket list. February and March are the best months for this encounter.
Marine life: Whale sharks (peak at Richelieu Rock), manta rays (Koh Bon, Koh Tachai), leopard sharks (peak), grey reef sharks, eagle rays. Harlequin sweetlips form huge schools at Richelieu Rock.
Gear tip: Same as January — 3 mm full suit. Bring a wide-angle lens for the whale sharks and a mid-range lens for the reef.
March: Continued Peak
March continues the peak conditions from January and February. Visibility starts to taper slightly (20–35 m) as plankton blooms increase — paradoxically, the plankton blooms that reduce visibility also attract the mantas and whale sharks. Water temperature warms to 28–29°C. Currents remain moderate, with some sites (especially Elephant Head Rock) running stronger as tidal flows increase.
Who should go: Divers who want peak wildlife encounters without the January-February chill. March water is noticeably warmer.
Marine life: Whale shark season continues through March. Manta rays are still reliable at Koh Bon and Koh Tachai. Leopard sharks common. The plankton blooms attract filter-feeding pelagics, making Richelieu Rock particularly active.
April: The Warm End
April is the last full month of the season before the closure. Water temperature reaches 28–30°C — the warmest of the season — and visibility is still excellent at 20–30 m. Early May is still accessible until around May 15.
Who should go: Divers who want the warmest water and who don't mind slightly reduced visibility. April is a good month for photographers who want the warm-water light quality.
Marine life: Whale sharks can still be sighted in early April. Manta activity continues. Turtle nesting season begins, so turtles are frequently seen on both the eastern and western sites. Leopard shark sightings decline slightly.
Gear tip: 3 mm shorty or thin full suit is sufficient. April water is the warmest of the season.
Early May: Last Call
The first two weeks of May offer warm water (29–30°C), reducing visibility (15–25 m) as the monsoon transition begins, and the last chance to dive before the closure. Conditions are more variable — some days are flat calm, others have building swell.
Who should go: Divers who can't make the October–April window and want the tail end of the season. Expect fewer divers but less reliable conditions. After May 15, the park closes until mid-October.
Understanding the Current
Similan currents are not uniform. Understanding the pattern helps you choose sites and plan your dives.
The east–west rule. The eastern side of every island faces the mainland and is sheltered. Sites like Hide-a-Way, Coral Reef, Miang Bay, and Paradise Reef consistently run mild currents suitable for Open Water divers. The western side faces the open Andaman Sea and draws the full fetch of currents — moderate to strong at sites like West of Eden, Deep Six, North Point, and Shark Reef.
Isolated pinnacles. Koh Bon Ridge, Koh Tachai Pinnacle, and Richelieu Rock are all exposed seamounts where current is dictated by tide, not by land shelter. Dive guides plan these dives around slack tide windows. Current can switch from zero to 3 knots within 30 minutes.
Drift diving norm. On the west side, drift diving is the standard procedure. Your guide drops you upcurrent, you drift along the reef wall or through boulder formations, and the boat picks you up at a designated exit point. Your dive briefing will always include the current plan — pay attention to it.
Thermoclines and Temperature Layers
Thermoclines are occasional below 25 m during the peak season months (January–April). When present, the temperature can drop 2–4°C in a vertical band of 3–5 meters. This is more common at deeper sites like Koh Bon Ridge and Koh Tachai Pinnacle.
The practical effect: if you're diving in a 3 mm suit and you drop below 25 m, an extra layer (hooded vest or rash guard) can make the difference between a comfortable dive and a shivery one.
Visibility Patterns by Site
Not all sites offer the same visibility. Here's how they typically compare:
- Richelieu Rock: 15–35 m — the widest range. Plankton blooms create the nutrient-rich environment that attracts whale sharks and mantas, but they also reduce clarity. Paradoxically, lower-visibility days are often the best for big animal encounters.
- Elephant Head Rock / West of Eden: 20–40 m — the best visibility among the inner Similans, thanks to the granite boulder topography that keeps sediment suspended and away from the dive path.
- Koh Bon / Koh Tachai: 20–40 m — generally excellent, though plankton blooms affect them similarly to Richelieu Rock.
- Eastern sites (Hide-a-Way, Coral Reef, Paradise Reef): 15–25 m — slightly lower due to wave action in the shallow environment, but still very good for photography.
Seasonal Marine Life Calendar
| Species | Oct–Nov | Dec | Jan | Feb | Mar | Apr | Early May |
|---|---|---|---|---|---|---|---|
| Whale shark | Rare | Occasional | Frequent | Peak | Peak | Occasional | Rare |
| Reef manta ray | Increasing | Good | Peak | Peak | Good | Good | Good |
| Leopard shark | Good | Peak | Peak | Peak | Good | Declining | Rare |
| Grey reef shark | Good | Good | Peak | Peak | Good | Good | Good |
| Green turtle | Year-round | Year-round | Year-round | Year-round | Year-round | Nesting | Nesting |
| Hawksbill turtle | Year-round | Year-round | Year-round | Year-round | Year-round | Nesting | Nesting |
| Harlequin sweetlips | Good | Good | Peak | Peak | Good | Good | Good |
| Barracuda | Good | Good | Peak | Peak | Good | Good | Good |
| Giant trevally | Good | Good | Peak | Peak | Good | Good | Good |
| Pygmy seahorse | Year-round | Year-round | Year-round | Year-round | Year-round | Year-round | Year-round |
Packing by Season
October–November: 3 mm full suit, dive light, SMB, reef-safe sunscreen. Bring an extra layer for the boat ride (windy conditions).
December–February: 3 mm full suit, dive computer, SMB, wide-angle lens for photography, reef-safe sunscreen. Boat rides can be cool in the morning — a windbreaker helps.
March–April: 3 mm full suit or shorty, dive computer, SMB, reef-safe sunscreen. Lighter packing overall.
May: 3 mm shorty, dawn is warm enough for minimal exposure.
All season: Dive insurance (mandatory — DAN or equivalent with evacuation coverage), NDL tables or dive computer, surface marker buoy. The Andaman sun is intense — a rash guard for surface intervals is recommended.
Weather Resources
Monitor these sources before and during your trip:
- Windy.com for real-time wind and swell forecasts in the Andaman Sea
- Thai Meteorological Department for regional weather patterns
- Park closure announcements via the Department of National Parks website or your dive operator
If a storm system is tracking toward the Andaman coast, liveaboard operators will adjust itineraries or delay departures. This is non-negotiable — respect the captain's weather calls. The Similan Islands have no protected harbors for large vessels, and a fast-developing storm in the Andaman Sea can turn conditions dangerous within hours.