I have not dove near Viti Levu but did around Taveuni so I don't have much info on your intended area. If I understand your location near Nadi, then the Mamanuca group is nearby with some sites to the South (Momi bay area possibly).
I suspect there are some posts about the Mamanuca and Nadi area in this forum or I think a poster or two that work in Fiji.
Getting to Taveuni from Nadi by air is about an hour to 1.5 hours. It depends on if there is a stop in Vanua Levu (Savusavu). Our inbound flight to Taveuni was direct but outbound had a stop at Savusavu, Vanua Levu. The Savusavu stop was not on our itinerary tho.
I hear there might be a ferry and that it leaves from Suva on the main island, Viti Levu. I have spotty information on the schedule and might be an overnight (and 7 hour trip). Suva btw is on the other side of the main island. If time is a constraint, I would fly. There are quite a few flights from Nadi to Taveuni