Hawaii is made up of 20 islands, the biggest which are Maui, Lanai, Molokai, Oahu and Kauai.  Here's the map.

Hawaii has year round warm waters and has been a popular scuba dive location for years offering some of the best dive sites in the world.  Diving in the Hawaiian Islands is more famous for its underwater landscape than for its coral reefs.  With strange underwater formations, such as lava tubes and caverns created by volcanic eruptions, the dive sites of Hawaii are pretty unusual.  Here are the highlights:

• Oahu - for excellent dive sites ranging from offshore lava formations and grottoes to plane and shipwrecks,
• Maui - most dive sites are located off the west coast and offers excellent diving for all levels of divers, including Molokini, Black Rock and Five Caves.
• Hawaii - most of the diving is off the leeward coast.  As the coast is sheltered from the trade winds by the large mountains conditions tend to glassy with visibility around 100ft plus. It is probably best known for night-time encounters with feeding manta rays.  See the YouTube video . . .

• Kauai - the Garden Island is known for its rugged and pristine diving. During the winter months large swells make the north shore generally inaccessible for diving, but during summer months and on calm days the north shore has some excellent locations.
• Ni'ihau - home to gigantic sea arches, the endangered monk seal, large pelagic fish and other rare species of fish.  This is a location for intermediate and advanced divers.
• Lanai - best known for lava formations which not only house a large variety of marine life but make for some amazing light effects.
• Molokai - along the entire South side of the island of Molokai lies the longest barrier reef in Hawaii. Seldom visited by divers, this natural sanctuary offers some of the best diving in the islands.

There are so many diving opportunities in these fabulous islands, too many to list and do justice but here are some tasters.

• Hawaii - Manta Ray Night Dive, Kailua Kona where mantas congregate in the early evening, and after dark, attracted to lights that are placed on the bottom of the ocean in about 30 feet of water. These lights attract clouds of tiny plankton and the plankton attracts the mantas. And you don’t need to be a certified diver to see the manta rays; this event is equally good for snorkelers.
• Oahu - Shark's Cove on Oahu for lots of arches and open-ended lava tubes, allowing deep light penetration and easy access. You'd be forgiven for thinking you might spot some sharks here but, no. The theory is that the rocks forming the cove look like a shark.
• Maui - US Navy PB4Y-1 bomber ditched off Maui in June 1944. That wreck, broken, with its wings flipped upside-down, is a technical dive (it rests in 200 feet of water).
• Molokai - Fish Rain for Hammerhead sharks, Tiger sharks, Monk Seals and the gigantic Whale shark.
• Hulopo'e Bay Marine Sanctuary on Lanai.

• Hawaii’s whale watching season takes place from late November to mid-April. There are numerous whale watching excursions throughout the state, many of which guarantee sightings and provide expert narration to heighten the experience.   Whales may be viewed from all the Hawaiian islands, but most of the whales congregate in the waters off Maui.
• In 2002, Hawaii joined Florida and the Cayman Islands and introduced a law banning commercial shark feeding in Hawaii state waters.
• Kauai and Ni'ihau are the least developed of the islands and have dive sites that are rarely crowded.