Shark Fin Reef

Shark Fin Reef runs like a half-submerged spine, its exposed crest suggesting the fin that named it, with two long faces of tilted granite slab sliding into sand past 30 m. The eastern side stacks hard-coral gardens and sweeping slab scenery; the west goes steeper and moodier. Blue-spotted ribbontail rays shuffle under ledges, garden eels stipple the deep sand, titan triggerfish excavate with their usual self-importance, and this southern corner remains one of the archipelago's better bets for a zebra shark flat out on the bottom. An unflashy, spacious dive that shows the Similans' scale without demanding anything exotic in return.

The moment you come for: Kilometer-scale granite spine, rays and garden eels on the sand, southern-sector calm

Topography: Tilted granite slabs and coral gardens dropping to sand

Current: Flow runs the length of the ridge and the dive becomes a one-way drift along whichever face is in the lee.

Entry: A long granite ridge southeast of island 3, its crest breaking the surface; often the first dive of northbound itineraries.

Hazards: Length of the ridge can string groups out in current; agree on the drift plan before rolling.

Conservation: Park rules apply; give nesting titan triggerfish their cone of territory in the warm months.

Nitrox: Worth it here: profiles sit in the range where enriched air meaningfully extends no-stop time (with the training to use it).