Western Rocky
Depth is the range from the top of the site to its usual maximum; "recommended cert" is the mainstream-agency level operators usually ask for, not a legal rule. Currents and visibility change daily — treat everything here as planning ranges.
Western Rocky, near the archipelago's southern edge, concentrates Mergui's character into one cluster of limestone islets. The centerpiece is a tunnel passing clean through the main rock, a torch-lit corridor where tawny nurse sharks rest in the recesses and lobster eyes glitter from the ceiling cracks; divers exit into blue on the far side feeling they have been let in on something. Around the islets, walls and boulder slopes hold tigertail seahorses, harlequin shrimp, ornate ghost pipefish, and frogfish in densities that keep macro guides busy, while trevally and rainbow runners work the outside water. Night dives here rank with the region's best.
The moment you come for: The through-island tunnel with resting nurse sharks, elite macro walls, strong night diving
Topography: Limestone islets with a through-tunnel, walls, and boulder gardens
Current: Current splits around the islets; the tunnel is dived on the quieter windows.
Know before you splash
Hazards: A genuine overhead passage with surge on some tides; save it for days when the water agrees.
Conservation: The tunnel's residents tolerate polite traffic; lights out of eyes and no blocking of the exits.
Nitrox: Worth it here — profiles sit in the range where enriched air meaningfully extends no-stop time (with the training to use it).
Who you'll meet
Frogfish
Harlequin Shrimp
Nudibranchs
Ornate Ghost Pipefish
Seahorses
Spanish Dancer
For photographers
Wide angle: ★★★★☆ · Macro: ★★★★★ — editors' 1–5 rating of what this site gives each style of shooting