Bula,
It will probably rain, but it will any time of year in the Tropics, just a little more often than in the winter. Other than during a tropical depression, rain is usually heavy but short downpours. December you might expect rain every 2nd afternoon, but also with periods of fine and calm weather lasting for a week or more. It is hot. 30+ C in the day and you will appreciate the AC at night. Water will be around 28 Deg C with only about 1 Deg change at 30m. Average winds are no greater than any other time of year, but of course you are in cyclone season which really means there is an elevated risk of tropical depressions forming which can be a bit of a low point for a diving trip. This year we, that is Fiji in general, would have lost maybe 7-10 days due to strong winds. Risk very small, but cost is high. The advantage of a liveaboard is you can get out of the way and dive somewhere else... probably. Have a great trip.
