Fiji consists of 322 islands (of which 106 are inhabited) and 522 smaller islets.  Here's the map.  The two most important islands are Viti Levu and Vanua Levu.  Viti Levu hosts the capital city of Suva and is home to nearly three quarters of the population.

The diving season is year round but the choice is often between good visibility and cool water or warm water and calmer seas with less visibility.  June to October is the dry season when the visibility is the best, but it's also at its coldest and the winds kick up.  Water temperatures also suffer at this time of year and can drop to the low 70s.

Things start to improve in November when the water warms up, the winds die down, and the plankton blooms, but that lowers the visibility.  but this offers the best chances of seeing large plankton eaters. The best time to go depends on your preferences: warm, calmer, cloudy seas, or clear but cold water.

• Astrolabe Reef on Kadavu in the Southern Islands (45 minutes flight from Nadi) has fabulous hard and soft corals and excellent marine life.
• Rainbow Reef in Northern Islands lies between Taveuni and Vanua Levu and has lots of scuba diving experiences including the Great White Wall (all white soft corals) and advanced drift diving through channels.
• Two separate barrier reefs between Viti Levu and Vanua Levu in the north east of Fiji are assessable from Nananu-i-Ra and the Lomaiviti Group. This is a rich area fed by tradewind currents with excellent marine life including mantas and hammerheads as well as bommies and pinnacles in the lagoons.
• Turtles nesting (Dec to March) 

• Some resorts chose to close down during February and March
• Targeted by the World Wildlife Fund as one of the earth’s most important marine ‘ecoregions’
• A migratory pathway and breeding group for whales; Fiji’s exclusive economic zone became a whale sanctuary in 2003.