The Andaman Sea offers some of the world’s most spectacular diving, from swimming alongside whale sharks at Richelieu Rock to exploring underwater boulder formations in the Similans.
Photos sourced from PexelsWith dozens of dive sites spread across 500+ kilometers of coastline, planning the perfect diving adventure takes more than a quick Google search.
Most divers researching Andaman Sea diving hit the same frustrations: conflicting information about the best season, confusion about which sites match their experience level, and difficulty comparing liveaboard options versus land-based diving. If vision correction is part of your setup, getting a diving mask for clear underwater vision fitted before you travel is as essential as booking your dives. Poor planning leads to either analysis paralysis or booking decisions built on incomplete information.
This guide cuts through the confusion with honest assessments of each major dive region, a seasonal planning calendar, and practical booking advice grounded in real logistics.
The Five Major Diving Regions
Southern Islands (Hin Daeng, Hin Muang, Koh Haa) sit roughly 80 kilometers southwest of Koh Lanta and deliver Thailand’s most dramatic wall diving. Both walls drop beyond 60 meters, with strong currents feeding enormous fish schools and reliable manta ray encounters from December through April. Koh Haa offers a sheltered alternative with caverns and swim-throughs suited to intermediate divers.
Similan Islands are the most popular Andaman Sea destination. Nine granite islands create a dual-character environment: calmer eastern sides host vibrant coral gardens for beginners, while western boulder formations at Elephant Head Rock demand solid buoyancy control. The Marine National Park closes mid-May through October each year.
Surin Islands and Richelieu Rock, located approximately 70 kilometers north of the Similans, offer pristine reef diving alongside Moken sea nomad communities. Richelieu Rock, a horseshoe-shaped seamount consistently ranked among the world’s top ten dive sites, is your best shot at whale shark encounters.
Koh Bon and Koh Tachai sit between the Similans and Surin Islands and function as dedicated manta ray cleaning stations from February through April. Both sites are exposed to open water swells and strong currents, making them better suited to experienced divers comfortable with drift diving.
Myanmar’s Mergui Archipelago offers expedition-style diving in pristine, rarely-visited waters. Access requires a liveaboard departing from Kawthaung or Ranong, and the season runs October through May.
Top Dive Sites: Honest Assessments
Richelieu Rock earns its worldwide reputation through sheer density of life. On a single dive you might pass a whale shark, watch a harlequin shrimp feed, observe a ghost pipefish against a sea fan, and count five nudibranch species without moving 10 meters. Whale shark encounter probability runs 40 to 60 percent from February through April, dropping significantly outside that window.
Despite a maximum depth of 18 meters, Advanced Open Water certification is strongly recommended. Unpredictable currents and four to eight boats per day during peak season require genuine buoyancy control.
Similan Islands work exceptionally well for mixed-ability groups. Eastern coral gardens at Christmas Point and Anita’s Reef suit Open Water divers returning after a break. Western boulder formations at Elephant Head Rock require 30+ logged dives and solid air management. Leopard shark sightings at Shark Fin Reef and Anita’s Reef remain among the Similans’ most reliable highlights, with 90 percent or higher sighting frequency reported throughout the season.
Hin Daeng and Hin Muang are not beginner territory. Strong current is the rule, Advanced certification and 30+ logged dives are genuinely necessary, and the walls drop beyond 60 meters into clear blue water. Manta ray encounters peak December through April at cleaning stations on Hin Daeng’s shallower ridge.
Koh Bon offers one of the most reliable manta ray locations in Thailand, with 70 to 80 percent encounter probability at cleaning stations during peak season. Koh Tachai’s notorious shifting currents make drift diving experience essential before attempting it.
Liveaboard vs Land-Based Diving
Photos sourced from PexelsA Thailand liveaboard is the only practical way to reach Richelieu Rock, the outer Surin Islands, and the Mergui Archipelago. Expect four dives per day including one night dive, compared to two or three on day trips. Over a four-day trip, that adds up to eight or more additional dives.
Seasonal Guide: When to Dive
November through December opens the season with 20 to 32 meter visibility, fewer boats than January onwards, and water temperatures around 27 to 28°C. This is one of the Andaman Sea’s best-kept secrets: excellent conditions with shoulder-season prices.
January through February brings the season’s peak conditions. Visibility reaches 25 to 35 meters, water clarity is exceptional at 26 to 27°C, and marine life encounters ramp up across all sites. This is the sweet spot if whale sharks are not your primary goal.
March through April is whale shark season. Plankton blooms that attract whale sharks reduce visibility to 15 to 25 meters, which frustrates wide-angle photographers but thrills wildlife divers. Water reaches 29 to 30°C and boats multiply significantly. Book six months in advance for peak March dates.
May through October sees the Similan and Surin Islands Marine National Parks close strictly. Southern sites around Hin Daeng, Hin Muang, Koh Haa, and Koh Lanta remain open year-round. Visibility drops to 10 to 20 meters, prices drop 30 to 50 percent, and you will share sites with far fewer divers. Avoid September and October if possible, as these months represent the monsoon’s peak intensity with the roughest seas and lowest visibility of the year.
Planning Your Trip: Practical Guide
Certifications and Logged Dive Recommendations
Open Water certification is the minimum for most sites, but Advanced Open Water is strongly recommended before visiting Richelieu Rock, Hin Daeng, or Koh Tachai. Honest logged dive benchmarks by site:
- Similan Islands east side: 10+ dives
- Elephant Head Rock: 25+ dives
- Richelieu Rock: 20+ dives with Advanced certification
- Hin Daeng and Hin Muang: 30+ dives with Advanced certification
- Koh Tachai: 40+ dives with drift diving experience
Nitrox certification adds real value on liveaboards where four dives per day push nitrogen loading. A 3mm wetsuit handles year-round temperatures comfortably. A custom prescription dive mask is essential if you wear glasses. Reading gauges, computers, or dive slates without clear vision at depth is a genuine safety issue, not a minor inconvenience. A dive computer is non-negotiable for multi-dive days, and reef-safe sunscreen is required within marine park boundaries.
Getting There
International flights connect to Phuket (HKT) from Bangkok (1 hour), Singapore (1.5 hours), and Kuala Lumpur (1.5 hours). Bangkok’s Suvarnabhumi (BKK) is the standard connection point from the US. Most liveaboards depart from Chalong Pier in Phuket for southern routes, or Thap Lamu Pier in Khao Lak for Similan and Surin routes. Khao Lak sits 1.5 hours north of Phuket Airport by road.
Most nationalities receive a 30-day visa exemption on arrival, with Thailand’s e-visa system simplifying extensions for 2026 travel. Check the Thai e-Visa portal for the latest entry requirements before booking.
A custom prescription dive mask from See The Sea RX is essential if you wear glasses, especially for reading gauges, dive computers, and underwater slates clearly at depth. Divers planning ahead can view the full collection before their trip. A dive computer is non-negotiable for multi-dive days, and reef-safe sunscreen is required within marine park boundaries.
Marine Life: Realistic Encounter Probabilities
Photos sourced from PexelsUnderstanding what you are likely to see at each site and when sets reasonable expectations and helps you choose the right timing for your trip.
Whale sharks at Richelieu Rock offer 40 to 60 percent encounter probability from February through April, dropping to 10 to 20 percent elsewhere and outside that window.
Manta rays at Koh Bon and Koh Tachai reach 70 to 80 percent probability at cleaning stations during peak season (February through April), dropping to 30 to 40 percent in the secondary October through January window.
Leopard sharks at the Similans appear with 90 percent or higher frequency throughout the season, making them one of the most reliably encountered large species in the region.
Sea turtles show up on 60 to 70 percent of dives across most sites. Whitetip reef sharks appear on 80 percent or more of dives. Hammerheads at Hin Daeng represent roughly a 5 percent probability, rare enough that spotting one feels genuinely special.
Beyond the headline species, Richelieu Rock’s macro density is extraordinary. Ghost pipefish, harlequin shrimp, porcelain crabs, and more than 50 recorded nudibranch species reward divers who slow down and look carefully rather than chasing depth or distance. Booking with operators that provide knowledgeable divemasters rather than just boat crew pays real dividends on macro-rich dives like this.
Choosing the Right Operator
Not all operators are equal, and the difference between a mediocre dive trip and an exceptional one often comes down to the quality of briefings, divemaster knowledge, and equipment maintenance rather than the sites themselves.
When shortlisting liveaboards, look for operators that carry valid DMTM (Dive Master Thailand Marine) certification, publish recent equipment inspection records, and maintain meaningful guest-to-divemaster ratios. Smaller boats with 12 to 16 guests typically deliver more personalised dive guidance than larger vessels running 30+ passengers. Divemasters who brief specifically on macro species at Richelieu Rock or cleaning station behaviour at Koh Bon signal a level of site knowledge worth paying for.
For land-based operators out of Khao Lak or Koh Lanta, PADI 5-Star Dive Centre status is a reasonable baseline filter, but read recent TripAdvisor and Google reviews specifically for comments on briefing quality and underwater guidance. Operators that provide species ID slates and conduct post-dive debriefs tend to produce better underwater photography and safer, more informed divers.
Questions worth asking any operator before booking:
- What is your divemaster-to-guest ratio?
- Do you provide Nitrox, and is it included in the price?
- What is your cancellation and weather delay policy?
- Which recompression facilities do you coordinate with in an emergency?
The answer to that last question, in particular, reveals how seriously an operator approaches safety logistics. The nearest hyperbaric chamber to the Similan and Surin Islands is at Bangkok Hospital Phuket, approximately 90 to 100 kilometers from Khao Lak. Reputable operators will have a documented emergency action plan and carry supplemental oxygen on every boat.
Photography and Visibility: Setting Honest Expectations
Underwater photography in the Andaman Sea rewards patience more than expensive gear. Wide-angle shooting works best January through February when visibility peaks at 25 to 35 meters and blue-water conditions prevail across most sites. Macro photography produces strong results almost any time of year, with Richelieu Rock and the Similans offering consistent subjects.
If whale shark encounters are your primary photographic goal, accept the trade-off upfront: March and April visibility of 15 to 25 meters means whale shark images will carry more particulate backscatter than you would get on a blue-water dive. A wide-angle lens or zoom rather than a fisheye will handle mid-range subjects more effectively in reduced visibility.
For beginner photographers, a compact camera in a compatible housing handles the Andaman Sea well. The diversity of subjects, from tiny nudibranchs at Richelieu Rock to schooling barracuda at Elephant Head Rock, makes the region a strong choice for developing a range of shooting techniques. Rent a camera housing from your liveaboard operator for a day before committing to purchasing your own setup.
Health and Safety Considerations
Diving is a physically demanding activity, and the Andaman Sea’s remote location makes preparation more important than in destinations near major medical centres.
Pre-trip medical checks are worthwhile for any diver who has not dived in more than 12 months, or who has experienced any cardiovascular, respiratory, or ear-related health changes since their last dive. Many liveaboard operators require a signed medical declaration before boarding.
Ear health is the most common complaint among divers in this region. If you have a history of ear infections or equalisation difficulties, consult an ENT specialist before travelling. Attempting to push through equalisation pain at depth is the single fastest route to a trip-ending injury.
Decompression illness remains a risk on high-volume dive days. Using Nitrox on four-dive days reduces nitrogen loading and extends bottom times, but it does not eliminate DCI risk. Conservative dive profiles, full surface intervals between dives, and avoiding alcohol between daily dive sets all reduce risk meaningfully. DAN insurance, as noted above, covers evacuation and recompression treatment and costs less per year than a single night’s mid-range hotel stay.
Sun and heat exposure on the surface between dives is underestimated by many first-time visitors. Pack high-SPF reef-safe sunscreen, a wide-brim hat, and a UV rash guard for surface intervals. Dehydration compounds fatigue and affects nitrogen elimination, so water consumption between dives matters more than most divers appreciate.
The Andaman Sea delivers world-class diving that competes with any destination globally. Success depends on matching your experience level, timing, and budget to the right sites and approach. Start by deciding your travel window and experience level, use the seasonal guide to identify optimal timing, then compare liveaboard versus land-based options based on your available diving days and target sites.
Book three to six months in advance for peak season. Verify your certifications meet the site requirements honestly before you arrive. Arrange dive-specific insurance before finalising your trip. And sort your vision correction properly before you go. Reading a gauge at 25 meters without a well-fitted prescription dive mask is not just inconvenient. It compromises your safety.
The Andaman Sea rewards divers who plan with honest information. Now you have it.
Citation note: Marine life encounter probability figures cited throughout this guide are industry estimates drawn from aggregated dive operator reports, liveaboard.com community data, and Scubaboard.com dive logs. No single peer-reviewed scientific survey underpins these figures. Where precision matters for your planning, request recent trip logs directly from operators before booking.
