From Undercurrent:
Fiji's weather presents a real mixed bag. The choice is often between good visibility and cool water or warm water and calmer seas with less visibility. June through October is the dry season when the water is the clearest, but it's also at its coldest and the winds kick up. Water temperatures can sink into the low 70s during this time of the year, making it necessary to drag out the full wetsuits. November brings a transition period. The water warms up, the winds die down, and the plankton blooms, lowering the visibility. By January and February, the water has warmed back up into the low 80s. The rains pick up and the hurricane season is on (December through March). Counting Tonga and Samoa, the area gets about five cyclones a year. It's a risky time to try to catch good diving weather. Because the winds kick up so much in February and March, some resorts pick these months to close down for repairs. During April and May, the wind, and therefore the seas, become calmer and the water remains warm, but the plankton bloom cuts down on the underwater visibility. Of course, this offers the best odds of seeing large plankton eaters. The best time to go depends on your preferences: warm, calmer, cloudy seas, or clear but cold water.