Phu Quoc's dive sites opened to recreational visitors in the early 2000s, but the island's coral ecosystems have anchored fishing communities for centuries. The An Thoi archipelago—fifteen limestone islets scattered across the Gulf of Thailand—shelters hard and soft coral formations at depths between eight and twenty-five metres, with visibility that regularly exceeds twenty metres during the dry season and holds above fifteen even through the southwest monsoon. The island's remoteness preserved these reefs longer than mainland sites, though warming events in 2016 and 2020 bleached patches of staghorn and table coral; recovery has been uneven, with healthy zones now concentrated around May Rut, Gam Ghi, and the northern flanks of Hon Thom. Dive operators base themselves at An Thoi Port, the southernmost harbour on Phu Quoc, where morning departures at 07:30 catch slack tide and calmer surface conditions. Most full-day itineraries include two tank dives separated by a surface interval on a beach or boat deck, with lunch served between sites. Beginner courses follow PADI or SSI protocols: a pool or shallow-water skills session precedes the first open-water descent, typically at a site with sandy bottom and minimal current. Certified divers access deeper walls and drift dives along the eastern edges of the archipelago, where nutrient upwellings attract schools of trevally, barracuda, and occasional reef sharks. Water temperature holds steady between 27 and 29 degrees Celsius year-round, warm enough that a three-millimetre shorty suffices for most divers. The Phu Quoc diving tour landscape divides between large-group snorkel safaris and smaller scuba-focused charters. The latter carry six to ten divers per boat, a ratio that allows divemasters to tailor descent profiles and bottom time to group experience. Equipment quality varies: established operators refresh regulators and BCDs on annual cycles and stock multiple wetsuit sizes, while budget outfits sometimes offer older gear with visible wear. Marine life diversity peaks between March and October, when plankton blooms draw manta rays and whale sharks into the outer channels, though sightings remain opportunistic rather than guaranteed. Coral health is best assessed by asking operators for recent site reports rather than relying on promotional images, which often date from pre-bleaching years.