@JennyFurious
Most of the good diving in Fiji (outer islands) are tied to resorts with their own dive operations. In the few areas where there are multiple dive operators, you can usually schedule dives with any of them, but they will have to come pick you up (often from a different island).
There are a number backpacker style resorts with dive operations both in the Mamanucas and the Yasawas, that might be your best bet unless you're planning on staying only on Viti Levu (where Nadi is) - which, other than the shark dive in Pacific Harbour (and some of the north coast) would mean you would miss much of the best diving in FIji.
If your goal is to visit Fiji and do a little diving this will work, however if your goal is to dive some of Fiji's best sites, etc. you are much better off picking a dive resort in the region you want to dive.