Sail Rock (Hin Bai)

Overview: The #1 Dive Site in the Gulf of Thailand

Rising vertically out of the open ocean halfway between Koh Tao and Koh Phangan, Sail Rock (Hin Bai) is an iceberg-shaped granite pinnacle whose tip rises 9 metres above the ocean surface before plunging vertically to a sandy sea bed at 35m to 40m.

Because it is the only major submerged structure for miles in open water, Sail Rock acts as a gigantic pelagic magnet, attracting massive aggregations of schooling pelagics, hunting kingsfish, and whale sharks.

The Famous “Chimney” Swim-Through

Sail Rock’s signature feature is “The Chimney”, a vertical natural swim-through inside the rock:

• Entrance at 18m: Divers enter a wide vertical chamber in the rock.
• Ascent Chamber: Look upward to see natural sunlight streaming through the exit.
• Exits at 12m & 6m: The chamber features two exits—a lower window at 12m and a shallow exit at 6m, which brings you directly onto the sunlit upper reef wall for your safety stop.
• Buoyancy Requirement: Strict neutral buoyancy is mandatory inside the chimney to avoid scratching yourself against the oyster-encrusted granite walls.

Marine Life & Pelagic Action

• Schooling Fish: Vast walls of chevron barracuda, pickhandle barracuda, yellowtail barracuda, and jacks circling the rock in massive schools numbering in the tens of thousands.
• Whale Sharks: Seen here year-round, but most frequently between March–May and September–October.
• Secret Pinnacle: Divers with Advanced certification and good air consumption can swim 50 metres east across the sand to the Secret Pinnacle (18m–32m), home to giant trevally and hunting Spanish mackerel.

Logistics & Depth Warnings

• Trip Type: Full-day boat excursion (2 hours each way from Koh Tao or Koh Samui) including 2 dives and lunch onboard.
• Certification Level: Open Water divers can dive the upper rock (0m–18m); Advanced Open Water is required for the full 35m perimeter and Secret Pinnacle.

Technical Dive Profile & Navigation

1. Entry & Descent Mechanics

• Descent Method: Direct giant-stride or negative back-roll from the boat, descending along the shot line to prevent surface drift.
• Ear Equalization: Equalize continuously every 1 metre during the descent phase to prevent sinus or middle-ear barotrauma.
• Compass Heading & Navigation: Set a primary heading toward the main reef structure. Maintain buddy contact within 2 metres throughout the descent.

2. Deep Section & Gas Management (14m to 35m)

• NDL Discipline: At depths of 30m, your No-Decompression Limit ticks down rapidly. Monitor your dive computer continuously.
• The Craig Rule: Divers certified to Open Water level (18m maximum) must stay on the upper reef shelf and never follow advanced divers down past 18m.
• Turnaround Gas: Signal your buddy and start your ascent when the first diver reaches 110 bar (or one-third of usable cylinder capacity).

3. Ascent & 5m Safety Stop

• Ascent Rate: Ascend along the reef wall at a controlled rate not exceeding 9 metres per minute.
• Mandatory Safety Stop: Complete a strict 3-to-5-minute safety stop at 5 metres (15 feet) before deploying an SMB and surfacing with a 360-degree overhead scan.

Marine Life & Habitat Zones (Biologically Verified)

• Apex & Pelagic Predators: Whale Sharks (Rhincodon typus), Chevron Barracuda (Sphyraena qenie), Queenfish (Scomberoides commersonnianus).
• Reef Fish & Keystone Species: Titan Triggerfish (Balistoides viridescens), Pink Anemonefish (Amphiprion perideraion), Giant Grouper (Epinephelus lanceolatus).
• Macro Biodiversity & Benthic Invertebrates: Janss Pipefish (Doryrhamphus janssi), Nudibranchs (Phyllidia varicosa), Banded Boxer Shrimp.
• Elasmobranchs & Marine Reptiles: Whitetip Reef Sharks, Leopard Sharks (Stegostoma tigrinum), Green Sea Turtles, Blue-Spotted Fantail Rays (Taeniura lymma).