Between 1933 and 1947, Koh Tao held prisoners rather than swimmers: the island served as a penal colony for detainees of the Boworadet rebellion, cut off from the mainland by seventy kilometres of open Gulf. A royal amnesty emptied it in 1947. The families who followed came by sailboat from Koh Pha Ngan and planted coconut across the granite slopes, and copra — not coral — funded the island for four decades. The Koh Tao Snorkeling Tour Private tours that now leave Mae Haad Pier trace the same sheltered western channels those cargo boats once used. Diving arrived in the late 1980s, when a handful of operators recognised what the plantation years had overlooked: warm, shallow, unusually protected water. The Gulf of Thailand is a submerged shelf here, rarely deeper than twenty-five metres between the islands, which keeps the reef within a snorkeller's reach rather than a diver's. By the 2000s the island had become one of the world's highest-volume certification centres. A private snorkeling boat tour is the quieter inheritance of that boom — the same reefs, without the tanks. That volume brought consequences. Bleaching events in 1998 and 2010 stripped colour from shallow Acropora across the archipelago, and community groups such as Save Koh Tao began coral nursery and mooring-buoy programmes to keep anchors off living rock. Koh Nang Yuan, the three granite islets joined by a shifting sandbar off the northwest coast and the anchor of almost every island-hopping route, now charges a 250 THB conservation fee, typically excluded from tour prices, to fund its own upkeep. The name itself — Koh Tao, Turtle Island — predates the dive shops by centuries, and hatchery releases have returned green turtles to the same bays since the 1990s. The geology explains the snorkelling. Koh Tao is a weathered granite dome, and its bays — Mango, Hin Wong, Shark Bay, Aow Leuk — are drowned granite bowls where boulders meet fringing coral in three to eight metres of water. Hawksbill and green turtles graze the seagrass at Shark Bay; blacktip reef sharks patrol its shallows. Japanese Gardens, on the sheltered flank of Nang Yuan's sandbar, holds staghorn and table coral in water clear enough to read the reef from the surface. Chumphon Pinnacle, the island's most celebrated dive site, sits eleven kilometres northwest and out of snorkelling range entirely — proof that the surface and the depths here are separate countries. Koh Tao Snorkeling Tour Private tickets exist because the geography rewards small boats: narrow entrances, limited anchorage, and reefs that thin out under crowds.