Fakarava is in French Polynesia.
Only come here if you are looking to escape from people and want to spend time in the water. There are less than 800 people on the island, so most everything you do will be on the water.
Get into the water. The lagoon is one of the clearest in French Polynesia and has little human interference. You will see many wild creatures. We took the day tour to Tetamanu (the pass at the south end of the lagoon). There were pleny of sharks (blacktip and gray reef) and the coral and the pass were amazing. Also try Scubadiving, its great here. There are several diveshops available. one of them is TopDive.
You will be limited in choices to where you stay due to the few people on the island.