Best time to dive Similan Islands

The national park closes for the monsoon from mid-May to mid-October (often opening around 1 November). Whale sharks and mantas are most likely February–April (whale sharks especially at Richelieu Rock, mantas at Koh Bon and Koh Tachai), with mantas from late December and both into the park's last weeks in May.

Peak season: December–April

Best upcoming months:7.9February 2027 7.8March 2027 7.7April 2027

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Overall diving, month by month

Balanced scoring · forecasts include the 2026/27 El Niño outlook

Confidence: Nov medium · Dec medium · Jan medium · Feb medium · Mar medium · Apr medium · May medium · Jun low · Jul medium · Aug medium · Sep medium · Oct low · Nov low · Dec low

What's good when

Each attribute scored 0–10 for every month

	Nov2026	Dec	Jan2027	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Pelagic & big animals	7	8	8	8	9	9	8	7	7	7	7	7	7	8
Macro life	6	6	6	6	6	6	6	6	6	6	6	6	6	6
Coral & reef health	6	5	6	6	4	4	4	4	5	5	5	5	5	5
Visibility	8	8	8	8	8	8	7	6	6	6	6	7	8	8
Water temperature	10	10	10	10	10	9	9	10	10	10	10	10	10	10
Weather & sea state	8	9	9	9	9	8	6	4	4	4	4	6	8	9
Operations & access	9	10	10	10	10	10	4	0	0	0	0	4	9	10

Climate outlook

How this year's forecast differs from a typical year

Water temperature +0.7 °C · Andaman Sea warms in the months after an El Niño peak (Nov 2026 – July 2027)
Coral & reef health -1 · Reefs still recovering from post–El Niño bleaching (Nov 2026 – December 2027)
Coral & reef health -2.3 · Post–El Niño bleaching risk, as in 2010 when dive sites were closed (Mar 2027 – June 2027)

Water temperature & visibility

Month	Water	Visibility
November 2026	28.8 °C (84 °F)	20–30 m
December 2026	28.4 °C (83 °F)	25–30 m
January 2027	28.5 °C (83 °F)	25–30 m
February 2027	28.6 °C (83 °F)	25–30 m
March 2027	29.1 °C (84 °F)	20–30 m
April 2027	30.2 °C (86 °F)	20–30 m
May 2027	30.1 °C (86 °F)	15–25 m
June 2027	29.5 °C (85 °F)	15–20 m
July 2027	28.9 °C (84 °F)	15–20 m
August 2027	28.8 °C (84 °F)	15–20 m
September 2027	28.7 °C (84 °F)	15–20 m
October 2027	28.6 °C (83 °F)	15–25 m
November 2027	28.5 °C (83 °F)	20–30 m
December 2027	28.0 °C (82 °F)	25–30 m

Marine life calendar

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
whale shark	●	Peak	Peak	Peak	●
manta ray	Peak	Peak	Peak	Peak	●							●
ghost pipefish	●	●	●	●	●	●	●	●	●	●	●	●
seahorses	●	●	●	●	●	●	●	●	●	●	●	●
moray eels	●	●	●	●	●	●	●	●	●	●	●	●

Peak = best odds · ● = regularly seen · blank = unlikely. Calendar months, every year.

About the diving

Difficulty: Intermediate
Current strength: 3 / 5
Luxury level: 4 / 5
Typical depth: 5–30 m
Max depth of interest: 40 m
Dive styles: Liveaboard, Day boat
Suggested certification: Advanced Open Water
Nearest airport: HKT
Snorkelling: Good
Known for: whale shark, manta ray, ghost pipefish, seahorses

Famous sites: Richelieu Rock · Elephant Head Rock · Koh Bon · Koh Tachai

Season notes & sources

The national park closes for the monsoon from mid-May to mid-October (often opening around 1 November). Whale sharks and mantas are most likely February–April (whale sharks especially at Richelieu Rock, mantas at Koh Bon and Koh Tachai), with mantas from late December and both into the park's last weeks in May.
The 2010 marine heatwave bleached shallow reefs around the Similan and Surin Islands; 18 dive sites were closed to recover.
Richelieu Rock supports several moray species and Boonsung Wreck is documented for numerous honeycomb morays. Resident presence is not an eel season: see the regional season note for monsoon and park/boat access.