Hi Deur,
I have been to Kadavu several times over the last 10 years.
Matava: Used to be great when it was run by Australians (Mark, Andrew and Andrea), but my personal feeling is that it has since gone significantly down hill with new ownership. There appeared to be no recent maintanence conducted on the resort, prices have gone up, new owners were rather antisocial and most of all they haven't developed any new divesites in the last 5 years. They seem to be more interested in gamefishing these days. Regardless of whether they haven't developed more dive sites, the diving is good. The best spots are in the passages and outside wall. However, bear in mind that the southern side of Kadavu is very exposed to the prevailing SE trade wind which (May-Oct) can be in excess of 25knts and when the wind is up, one can't dive on the outside. They do have a very good manta site, where one can watch 10+ pacific mantas doing feeding loops (wicked!). My favourite dive site there is Naiqoro passage. If your an experienced diver, go to the bottom of the passage (~50m) and checkout a field of people-sized purple and orange soft coral trees. Also potential for BIG pelagic action (eg. silvertips, tigers, walu, wahoo, eagle rays). Awesome spot! Worth going to Kadavu just to dive Naiqoro.
I have also been Naigaini (surfing), but haven't dived there. Diving potential looks good. Unlikely to have killer softies, as there aren't too many passages nearby. People who dived with Dive Kadavu had a good time, but were concerned about the condition of their rental equipment. In addition, there was an accident (AGE) involving another diver while they were out on the boat.
Go, you'll love it. Good luck with the wind. Trade wind tends to do weekly cycles ie. blows like hell for a week, then backs off for a day or two, then wind for a week...etc