Koh Seh covers 7.5 square kilometres — 4.4 kilometres long, between 1.8 and 4.4 wide — and carries fifteen kilometres of coastline for a landmass you could cross on foot in an afternoon. That ratio is the whole description: the shape is all inlets, points and tidal creeks rather than any clean outline. It lies a kilometre and a half south of Koh Thmei and about nine kilometres off the mainland at Sihanoukville's Ream commune, and it marks the southeastern limit of Ream National Park, established in 1993 across 210 square kilometres, 150 of them land and 60 sea. Less than four hundred metres to the southwest sits Koh Ky, an islet small enough that most charts give it a dot and leave it there. The French knew Koh Seh as Île à l'Eau, a name that now survives only on old maps.

Southeast of the island the water opens into a sound 4.3 kilometres wide, and on the far side of it is Vietnam. Phú Quốc is one of the stranger outcomes of colonial cartography — an island lying far closer to Cambodia than to the Vietnamese mainland, and Vietnamese anyway, because in 1939 a French governor-general ruled a line across these waters and the line outlived the empire that drew it. The boundary stayed contested through independence and the wars that followed, and was not physically demarcated on the ground until 2012. None of that is history to the fishing crews working these shallows. It is a line you stay on the correct side of or explain afterwards, drawn through water that fish, turtles and dugongs cross without asking permission from either government.

Mangrove fringes nearly all of Koh Seh's shore, and that is the island's actual job. Stilt roots slow the water, drop sediment and build the murky shallows where juvenile fish, crab and shrimp can grow without being eaten in the first week; fisheries along this entire coast depend on nurseries like it. Ream National Park exists in large part to keep them intact. Within its boundaries live rhesus macaques in the forest, dugongs and dolphins offshore, marine turtles on the seagrass, and Sarus cranes in the wetlands — a list that holds together only as long as the mangrove does. Nearly thirty thousand people live in the five communes that overlap or border the park, across roughly 5,500 households, and that population climbs by about three percent a year. The park can support a limited level of use. The demand on it is not similarly limited.

So Koh Seh sits where it has always sat: no village, no pier worth the name, no recognition, quietly absorbing the credit and the blame owed to an island sixty kilometres east. Its neighbour Koh Thmei, a kilometre and a half north, has been sliced into development zones on paper and assigned to a casino company. Phú Quốc, across the sound, has an international airport, a safari park and a population past 150,000. Koh Seh has 7.5 square kilometres of mangrove, forest and coarse sand, and the singular advantage that nobody has yet drawn up a compelling reason to change it. On this coastline, in this decade, being mistaken for somewhere else may be the most effective protection an island can get.