Island of Hawai'i, Keauhou Bay:
Just south of Kailua-Kona, this is a fantastic night dive for meeting manta rays. Floodlights shining into the water (from the hotel on shore, as well as from dive boats that flock here) attract plankton, which bring the mantas in to feed. Accessible from shore or by boat.
Island of Hawai'i, Puako Reef:
This rocky reef along Puako, north of Kailua-Kona, is the kind of place where you can have a dozen different experiences from the same entry-point. Sea turtles and reef fish abound, and when you finish diving, the picture-postcard Hapuna Beach is just minutes away.
Island of Hawai'i, Punahonua o Honaunau (Place of Refuge):
The reef here stretches down to meet a sandy bottom at 30 metres/100 feet, where you can sometimes find garden eels waving in the current. The reef itself teems with fish and sea turtles, and you can visit the Place of Refuge between your dives. Stay alert to the occasional motor boat, as locals often put in small craft here.
Island of O'ahu, Makaha Caverns:
Look closely at the reef (but keep your fingers out of the pukas, or holes) to find octopus, moray eels, fan corals, and intriguing invertebrates. Keep an eye out also for reef sharks and eagle rays. The caverns (lava-tube formations with swim-throughs) and nearby Mahi wreck provide interesting nooks and crannies for marine life to inhabit and divers to explore.
Island of O'ahu, Hale'iwa Wall:
In addition to a fascinating wall-dive (steep drop down to 27 metres/90 feet), divers can also see a natural turtle-cleaning station at work. Up to two dozen turtles at a time may float here while reef fish nibble algae off their backs. Look for scorpionfish,, eels, and invertebrates as well as reef fish.
Island of Kaua'i, Tunnels Reef:
Kaua'i's reefs are among the islands' healthiest, perhaps because they are the oldest. Tunnels Reef writhes with marine life, including octopus and myriad other invertebrates, largely undamaged coral formations, sea turtles, and many species of reef fish. Because of rough currents and wave action in the winter, this site is only recommended from April through October.
Island of Maui, Molokini Crater's back side:
This crescent-shaped crater harbors a shallow sheltered reef in its interior, but it's the back side that's truly memorable. A dive boat will drop you some distance from the crater wall and allow you to angle down through open water to the sheer face. If you dive at the right time of year, humpback whale songs will echo off the cliff while you drift-dive with the current along the wall. Bring a small dive light to illuminate those critters in the crevices.
Island of Lana'i, Fish Rock:
Lana'i dives are generally undertaken by boat from the neighboring island of Maui. Lana'i is home to the Hulopo'e Bay Marine Sanctuary, and Fish Rock (true to its name) is a veritable rainbow of reef fish. The nearby Cathedrals (lava-tube formations) are also worth a visit if you're up for a multi-dive day.
Island of Moloka'i, The Cove:
In the winter months, this protected cove is a bonanza of marine life, notably large formations of antler coral, butterfly fish, and (on night dives) manta rays and several species of lobster. Watch for currents if you stray out of the protected cove itself.
Island of Ni'ihau:
The island itself is owned and populated by native Hawai'ians and not open to tourists, but divers can take a boat from neighboring Kaua'i for the marine experience. The Hawai'ian monk seal may be spotted here, as well as dolphins and (thanks to the nearby seabird sanctuary) a wide variety of endemic seabirds.