Picture here isn't Fakarava, but sure is a nice example of atoll structure. All reef, no island. Coral reefs generally offer great diving. Normally, you find them in the water ringing tropical islands. But an atoll... is reef, without island. What happened? Where did the island go?
Pacific tropical islands generally start their lives as volcanoes. If the volcano erupts under water long enough, it builds an enormous subsurface mountain... that may eventually break the surface. Voila, a volcanic island. (Actually, volcanic mountaintop.)
Over aeons, that volcanic mountaintop will stop erupting, start eroding and forming soil, be colonized by bugs, birds, plants, etc and become a tropical island.
In the clear, warm water offshore, coral reefs begin to form and eventually build a ring around the island. The ring will usually be broken where rivers flow off the island into the ocean... because the silt involved retards coral growth in that area. If you are a human who wants to colonize the island with your friends in your canoe, you look for that break in the reef and paddle like crazy through it for the calm water inside.
So, all seems well. Tropical paradise, etc. But give it another few million years... and the central volcanic mass may have both eroded dramatically and sunken downwards into the crust of the earth under its own weight. One day, the island disappears again below the surface... and keeps sinking slowly.
The coral reef though is a living thing, and it continues growing. That fringing reef now sits atop the sunken volcanic island, and keeps building towards the sun while the island keeps sinking. If reef growth and island subsidence hit an equilibrium point -- known as the Darwin Point, after guess who -- an atoll results.