Phuket's offshore archipelagos shelter more than 200 documented coral species across a network of national marine parks that draw 1.8 million visitors annually. The Similan Islands, 84 kilometres northwest, sit atop granite boulders deposited during the Cretaceous period, now draped in staghorn and table coral gardens that descend to 30 metres. Phi Phi's limestone cliffs, formed from sedimentary deposits 260 million years old, frame Maya Bay, where the 1999 closure for ecosystem recovery preceded a 2018 reopening with daily visitor caps of 375. Bamboo Island's crescent beach marks the northern boundary of the Phi Phi National Park, established in 1983 to protect nesting sites for hawksbill turtles and breeding grounds for blacktip reef sharks. Snorkeling day trip phuket itineraries depart Phuket Marina between 07:00 and 09:00, when morning thermoclines concentrate visibility at 15 to 25 metres before afternoon surface currents reduce clarity. Speedboat routes cover 40 to 170 kilometres round-trip, with hull designs introduced in 2014 cutting transit times by 30 per cent compared to longtail alternatives. The Khai Islands—Khai Nai, Khai Nok, Khai Nui—lie nine kilometres from Phuket's eastern shore, their shallow coral platforms at three to eight metres depth supporting parrotfish, sergeant majors, and occasional green sea turtles. National park regulations require the 400-baht conservation fee for foreign adults at all marine protected areas, collected on-site by uniformed rangers who also enforce no-touch coral protocols. Operators now carry reef-safe sunscreen samples and provide wetsuit rentals after 2021 Thai Marine Department guidelines linked oxybenzone residues to 14 per cent bleaching acceleration in monitored zones. Phi Phi tours typically anchor at three sites—Pileh Lagoon's enclosed basin, Monkey Beach's rocky headland, and Maya Bay's restored coral nursery—while Similan itineraries prioritise dive sites Eight and Nine, where whale shark sightings peak between February and April. Bamboo Island stops include the eastern reef wall, where current patterns create nutrient upwellings that sustain schools of fusiliers and trevally. Most vessels return to Phuket Marina by 17:00, aligning with the 18:00 facility close. The Andaman Sea's position along the Sunda Trench generates seasonal monsoon patterns that divide the snorkeling calendar: November through April brings two-metre visibility advantage and one-metre calmer swells compared to the May–October southwest monsoon, when tour schedules compress to morning-only departures.