Unfortunately, I provided the torrential rain comment previously so I'll elaborate on my experience instead so you can evaluate.
We went to Taveuni on May 29 through June 6, 2004. The day we arrived it was beautiful. Hot. Somewhat humid. Next day, got up, another nice day. We rented a house on the beach right by the airport. The house was OK, but various features were not as advertised, most importantly the one person "staff". That night, the rain came in. Strong wind. Torrential rain. Really blowing. It rained with little break for the next six days straight. No sun, just solid cloud. At the same time, I got sick from some idiot on the plane out so I laid around the house for a few days trying to get better to salvage the rest of the trip. It just rained and rained. Windy like it was going to blow the roof off at night. It was difficult to go out because with the wind blowing, you just got soaked through. Newlywed neighbors who did attempt to visit the waterfalls said it was an impassable torrent. Multiple times, cells came through that would turn into white outs while they went over. We couldn't see the beach down below the house. The day we left, it was beautiful again, and the strait appeared as a lake, gently lapping. Perhaps it was me, however, we flew through plenty more storm clouds on the way out.
We went with Swiss Fiji Divers out on their smaller boats. The crew was great but conditions were pretty rough. As long as you didn't stop, you were fine.

A story in itself, their small boats were enclosed so that the exhaust would fill the passenger area. Despite Bonine and no previous seasickness, my wife was twice reduced from happy diver to a sputtering mess from the carbon monoxide on the way to the dive. The divemaster always stood in the open at the back of the boat to get the most clean air. I was tall enough to get my head over the tanks along the side out into clean air. Ridiculous.
If you research Taveuni, you'll find that it has a large mountain running down the middle. On the airport side, it gets some of the most rainfall in the world. Somewhere I read that it gets some large number of feet of rain per year. Still, I wanted to go to Taveuni for the strait, so we went.
What I'm saying is: you have a high probability of running into rain which could be heavy and for multiple days. It won't stop you from diving but it may not be the sunny paradise you might be expecting.
maddmike:
How often is the Somosomo / Taveuni weather severe? On a tropical island, I expect some rain. But I've heard from several people who had "days of driving, horizontal, torrential rain" on Taveuni. Any idea how frequent that is? We'll be in Fiji late May-early June.
How does the diving quality compare between Taveuni/Somosomo and/or Kadavu/Astrolabe with the Mamanucas? I've gotten the feeling that you can go diving in the Mamanucas, but it's not nearly as good as at the wetter islands.
Thanks.