You are correct - vitually no beaches anywhere on Ovalau to the best of my knowledge. To the contrary Naigani has outstanding beaches - particularly round the point on the leeward side of the island at Picnic Beach....stunningly beautiful. The beach at the resort is also very good, with excellent snorkelling wrapping around the small island offshore. Naigani is really a good option and I'd prefer a week there to anywhere on Ovalau, but Ovalau has some interesting culture to experience. Maybe try to get to Lovoni, the only village in Fiji never defeated in battle and even today, a proud and strong people.
If you go to Naigani you would have to either fly to Suva (local flights are reasonably expensive) or save a lot of dollars and bus around the south coast. It really is quite a scenic journey, though a long haul - at least 5 hours from Nadi airport to Natovi landing (departure point for Naigani.... If you end up going to Naigani I would love to hear about it, as I have heard very few reports over the last few years. I had one expereinced dive buddy go there and tell me "Wiggsy! Its the best damned diving in Fiji and nobody goes there!"
Don't know if that is indeed the case but my first dive trip there in 2003 saw 30 metre visibility every dive and masses of fish life and glorious soft corals....Friends saw my images and compared them to the best of The Red Sea. The only thing you won't get is lots of large pelagics. More like a dozen or so Trevally (Jacks) and the odd school of Barracuda. Its all pinnacle diving - you just spiral around huge columns of coral and the tops of the pinnacles are so full of colour and life that its like diving in an aquarium!