Oʻahu

Most diving here is a boat day out of Kewalo Basin Boat Harbor, a short drive from the Waikiki hotels, or from the Kai marina at Hawaii Kai further east. Operators run daily two-tank charters and the sites sit close in: the Sea Tiger is ten minutes off the dock, Koko Craters about the same from its own marina. Shore diving runs alongside it, from the protected inner reef at Hanauma Bay to the steep scramble down to Halona Blowhole, but heavy boat and watersports traffic makes some south-shore entries unsafe to swim out to, so shops put those dives on a boat instead.

The seabed is young lava and reef flat, and the terrain steps rather than slopes. Ledges such as Skimin's Reef drop from 7-12 m to 18-24 m and run for half a mile. Southeast of Hawaii Kai the sea cliffs carry on below the waterline into walls that start around 18-27 m, with caves carved into the lava at Spitting Cave and drift dives along the boulder ledges of Split Rock to 49 m. Green turtles are the constant, whitetip reef sharks work the wrecks, and about 30% of Hawaii's reef fish are endemic; Hawaii holds more endemic tropical fish species than any region except the Red Sea.

Three state Marine Life Conservation Districts cover the main dive zones: Hanauma Bay, Waikiki and Pupukea, and collecting material inside them is restricted. Hanauma Bay was the first, established in 1967, and is also a state Nature Preserve; non-residents now need an online reservation, daily numbers are capped at 720, and the park closes Mondays and Tuesdays to rest the reef. The barge wrecks off Kahala are dilapidated enough that divers are asked not to touch them, and the Kahala Barge is not safe to penetrate.

It suits divers fitting wrecks and turtles around a city trip rather than building a holiday around the diving: airport, hotels and harbours all sit within a few miles of each other, and PADI alone lists over 300 named sites around the island. It also asks for honesty about the geography. The dependable, operator-served diving is the south shore between Waikiki and Hawaii Kai; the Mahi off Waiʻanae and the Nashua tug near Pearl Harbor lie 20-45 km away on a different coast and sail from other harbours, and Pupukea on the North Shore is calm only from about May to October. Anyone wanting one continuous reef coastline rather than sites scattered across several shores is better served by Kona.