I produced a CD ROM about Fiji many years ago and had the opportunity to dive all the areas around Fiji as far north as Yasawa, south to Kadavu, east to Matangi and west to Tokoriki. What I found was that the diving was not the same in all areas and the amazing soft coral shots you see in the Fiji ads and magazines come mostly from the area between the two big islands. Some of the places I expected to see great diving, just wasn't that great in my opinion. They had some soft coral, but it was very small and scatted among the the hard coral.
Since then Fiji suffered coral bleaching which caused significant damage. The good news is that my last trip to Fiji, a little over a year ago, I was amazed how great the reefs looked. I too dived with Kai Viti and found them to be one of the best dive operators in Fiji. Great boats, very responsible and takes you to those areas where you'll see soft coral as far as the eye can see. I think I was there just a couple of weeks after Eric and James. Then I spent five days at Castaway Island in the Mamanucas and I was very impressed with the diving there too. We visited soft coral pinnacles and very healthy hard coral bommies. I didn't expect that, but was very happy to see it.
If I had to do it again and time or money were not a factor, I'd get on a live-aboard and go back to the area between Gau and Namena, which was my favorite area, as well as the Vatu-I-Ra Passage. The north end of this area is vistited by Kai Viti on special trips (pay a little more, but worth it).
I don't think rain is going to be a big problem during this time. It gets heavier in Feb and March and the water gets much warmer but its not bad at all in Dec. It usually rained at night and was sunny during the day. You can see a very nice chart of annual water temps on the Nai'a web site. Here is also a chart of average air temp and rainfall.
Have a great trip!
Bonnie
Almost, James...KaiVitiDivers.com