Dive With Us in The Similan Islands: #7 Koh Bon

Our journey continues as we sail north from the Similan Islands towards Koh Bon, a lonely limestone island about 20 kilometers away from Similan Island number 9. The short crossing is always full of excitement. Pods of dolphins frequently ride the bow wave at the front of the boat, exciting divers as they leap alongside. And on rare occasions, the open water offers glimpses of passing whales — false killer whales, melon-headed whales, or even the occasional sperm whale traveling through the Andaman Sea.

The island appears in the distance as we sail closer. Although Koh Bon is not one of the nine granite islands of the Similan archipelago, it still belongs to the Similan Islands National Park. Its character sets it apart from its neighbors: instead of towering granite boulders, Koh Bon is composed of porous limestone. Over thousands of years, acidic rainwater and seawater have carved through the rock, forming caves, overhangs, and dramatic walls both above and below the surface.

Time to Dive

Our drop point is in the south bay, just under the “waterfall,” where waves tumble down the rock face from the swell on the other side. Just to the right is the natural hole that cuts through the island, visible above water. From the boat it looks like a window into the sea, a reminder of how water and time have sculpted Koh Bon’s limestone. It’s a landmark we always point out to divers, both beautiful and useful for orientation.

Rolling in, I descend into clear blue, using the wall as a reference. The visibility stretches wide, and below me the limestone wall cascades into the depths. Massive boulder corals rise from the ledges, their rounded shapes creating shelter for schools of reef fish. Delicate lettuce corals and table corals stretch outward catching the sunlight and casting shadows below. Encrusting corals add layers of texture and color, clinging tightly to the limestone. Together, these corals form a living foundation that shapes the reef and supports its marine life.

I pause for a moment, hanging in the water column. The bay feels calm, almost sheltered compared to the open sea outside. Schools of fusiliers stream past, flashing silver and yellow in the sunlight.

We begin to drift slowly along the limestone, keeping the island to our right shoulder. The wall is alive with detail. Coral whips curl out from crevices, their branches covered in tiny polyps, while parrotfish crunch loudly at the reef. I shine my torch into a crevice and spot a banded cleaner shrimp, its red and white antennae waving as I pass. 

Soft corals cover the ledges in pinks and yellows, swaying in the surge. The swell is gentle now, but I can feel its rhythm — a steady back-and-forth motion that rocks the entire wall. At times, however, the movement can be far more powerful. Even distant tropical storms leave their mark here, with tailwinds reaching across the Andaman Sea and driving strong surges against Koh Bon’s limestone cliffs.

Koh Bon’s West Ridge

The wall continues down to around 20 meters, then slopes gradually into sand at 30 meters. Around 50 meters west from our drop point, the limestone stretches out into the deep, leaving the island behind at the surface. Suddenly I can see Koh Bon’s infamous west ridge forming beneath me, it’s dramatic topography adding an eerie feeling to the dive.

There are also smaller ‘ridges’ — sections of limestone rock that extend underwater into the depths from the island — further north, and they can be done together when the current is strong as a gentle drift dive. We often do multiple dives here, with different drops each time depending on the current’s strength and direction.

As we pass the wall, the top of the limestone begins to dip under the surface. Here, the dive site is completely unprotected from the currents. Sometimes the current can be mild, and other times it can easily take you away from the site, so it’s important to stay behind your guide and follow their lead. The safest approach is to stay low, keep streamlined, and use the ridge itself as shelter. By positioning your body behind or close to the rock, you escape most of the current’s force while also protecting the fragile corals from accidental contact. Never hold onto corals or rest your fins in the reef.

We begin to follow the top ridge at around 25 meters, staying close to the rock to avoid the current. The limestone slopes into the depths on either side. I scan for nudibranchs and peacock mantis shrimp, before I notice a large silhouette taking shape in the distance…

Oceanic Visitors

Koh Bon is most famous for its pelagic encounters. Among these, none is more iconic than the oceanic manta ray. Oceanic manta rays are the largest of their kind, with wingspans up to seven meters. They can be recognized by their white shoulder patches, triangular fins, and distinctive spot patterns on their underbellies that are unique to each individual. Unlike reef mantas, which tend to remain local, oceanic mantas are true pelagic travellers, crossing vast distances in open water, yet they return again and again to specific sites like Koh Bon.

The reason lies in the ridge itself. Its location and orientation create ideal cleaning stations. Cleaner wrasses, butterflyfish, and angelfish gather along the top edges of the ridge. Mantas use these spots like underwater spas, hovering gracefully as the smaller fish pick parasites from their skin and even from their mouths.

We hold steady as the silhouette emerges. One manta glides over the ridge, followed by two more circling in shallower water. They hover gracefully above, twisting through our bubbles as cleaner fish remove parasites from their skin. Their calm presence is mesmerizing; despite their size, they move gently, slowly approaching divers with an inquisitive look.

Manta Ray Etiquette

Encounters with mantas demand respect. Divers should stay low, avoid chasing, and give them space to approach naturally. Flash photography and sudden movements can startle them, often ending the encounter. In addition, touching must never happen. Mantas are protected by a thin mucus layer that shields them from parasites and infection. Contact removes this barrier, leaving them vulnerable and stressed. By remaining calm and maintaining distance, divers protect both the animal and the experience for everyone in the water.

In past seasons, manta sightings at Koh Bon were predictable, peaking from late February to March. Recently, however, encounters have become less regular. Possible causes include climate-driven shifts in plankton availability, changes in ocean currents, and illegal fishing pressures that disrupt their migration. These factors mirror broader challenges across the world’s oceans, underscoring the importance of conservation.

Ascending the Ridge

The ridge is deep, and mantas are captivating — but divers must constantly monitor their computer and no-decompression limits. It’s easy to lose track of time while watching them, yet exceeding limits here is a very real risk. Awareness and discipline are essential, even during the most unforgettable encounters. 

We begin to shallow up, continuing right shoulder over the ridge. The current is pushing against us so I kick hard for a few meters to reach the other side. Here I can relax, sheltered in front of the ridge while the current goes up and over.

On the north side of the ridge is a sloping reef, continuing along the island for a few hundred meters before meeting the smaller north ridge. In the sandy bottom just at the edge, it’s common to see resting white tip reef sharks and sometimes even leopard sharks.

The West Reef

The reef here is barren. Broken corals cover the seafloor — a place that was once thriving with life and colour. Before the bleaching, this section was dominated by branching staghorn corals and wide table corals, creating intricate structures that supported schools of fish and countless invertebrates. These fragile corals were the first to disappear during the global El Niño bleaching events in 2010 and later in 2016. In contrast, larger boulder corals proved more resilient, surviving the heat stress but unable to rebuild the reef’s former complexity. Most areas in the Similan Islands National Park have recovered well, but this patch continues to struggle.

Several factors contribute to the lack of recovery. The substrate here is loose and unstable, offering little for new coral larvae to attach to and grow. Repeated wave action and surge continually shift the rubble, breaking apart young colonies before they can establish. Sedimentation also plays a role, smothering fragile recruits and preventing photosynthesis. In addition, rising sea temperatures and irregular bleaching cycles create further stress, reducing the resilience of any new growth. Therefore, the west reef of Koh Bon is very unlikely to recover without intervention like coral restoration.

Even though the reef is mainly rubble, the rubble remains shelter and protection for marine life. I search around, looking beneath the reef for shrimps, nudibranchs and hiding juvenile oriental sweetlips and angel fish.

I notice something moving, perfectly camouflaged but given away by its movements. An octopus is searching through the reef for food. I admire it for a moment as it lucidly weaves between the reef before sinking deeper into its crevices.

These animals are highly intelligent, known for solving problems and manipulating objects with precision. Their camouflage is remarkable; octopuses can shift color, texture, and even body shape within seconds. Each of their eight arms are lined with powerful, sensitive suckers, capable of gripping, tasting, and exploring their environment independently. Sightings in the Similan Islands are common, yet every encounter feels special.

Koh Bon Pinnacle

Koh Bon Pinnacle lies just off the island’s west reef, a deep, submerged limestone mountain that begins at around 18 meters. Divers can reach it by swimming west from the reef, a journey that usually takes five to ten minutes. However, the swim is exposed and can be easily missed if the current runs against you. The pinnacle itself is striking, covered in soft corals and often attracting schools of fish along its slopes. Despite its beauty, we do not dive here often. Strong currents and poor visibility can make it challenging, and for safety, we only attempt it in the right conditions as direct drops on the pinnacle can be difficult. 

Back to The Ridge

We turn around and head left shoulder back to the ridge. Now we are shallower, and can explore a different part of the wall from a different perspective. As we approach the ridge, we come across a large coral bommie covered in sea fans and glass fish that shimmer against the shallow light. I briefly check under it for spiny lobster, before hopping to the smaller bommies by the ridge to look for ghost pipefish.

We make our way back over the ridge, looking up towards the surface as the waves and surge dance against the island above. Following the wall, we look again into the blue for things passing by — dogtooth tunas, Napoleon wrasse, or maybe the mantas from earlier are still here and are joining divers for their safety stops.

Exploring the South Bay

We head further east into the bay, and the wall begins to end as it shallows up to join the reef. The bay is dotted with coral bommies, each one sheltering it’s own ecosystem. I begin to look under them for hiding stingrays, or even some white tip reef sharks resting before their evening hunt.

Most sharks must swim constantly to breathe, relying on ram ventilation as water flows across their gills. White tip reef sharks are one of the few shark species that do not need to keep swimming; they use buccal pumping, a method where muscles in the mouth actively draw in water. The water is then forced over the gills, allowing them to extract oxygen even when lying still. This adaptation explains why divers so often find them resting on sand or beneath coral bommies during the day.

The dive draws to an end. Batfish approach curiously as our guide deploys the SMB, its orange tube rising toward the surface to mark our position. The bay is busy with dinghies and dive boats, so staying close to the SMB during the safety stop and while ascending is essential. We complete our safety stop surrounded by fish, bubbles shimmering upwards, before ascending into sunlight.

Koh Bon rises behind us as the dinghy approaches, the memory of soaring mantas and limestone ridges lingering. This dive has shown us a very different face of the Similan Islands National Park — one shaped by limestone rather than granite. Koh Bon feels unique, a place where geology and marine life combine in unexpected ways.

Like what you read? share it with your friends!

Next en route on board MV DiveRACE Class X:

Our journey continues further north. We leave the limestone island behind and return to something more familiar, yet even more dramatic…

Want to read more? Take a look at our latest posts!

• DiveRACE Safety Series: Diving Practices & Underwater Awareness
• Purpose-Built Workboats: The Support Heroes of MV DiveRACE Class X
• Symbiosis in The Similan Islands: 10 Intriguing and Disturbing Examples
• Acropora Corals in Thailand: The Science Behind Our Restoration Work
• Dive With Us in The Similan Islands: #12 Boonsung Wreck