I produced two CD ROMs about the Fiji Islands and have been there seven times over the years. I had the opportunity to visit 24 different resorts & hotels and three different live-aboards. Here's what I learned...
Soft coral gardens are not found everywhere in Fiji, even though some areas claim to be loaded with it and often times use soft coral photos from other areas of Fiji to advertise in their brochures and websites. I had heard so much hype about the "soft coral capital of the world" I was very disappointed when I got there until I actually found the best diving.
The time of year you choose to visit has a big effect on water temps and visibility. Warm water means reduced vis. You'll get the best vis in July and August, but the water is colder. Because you're also looking for surfing, you're going to give up some of the best diving in Fiji. The best surfing and best diving are not side-by-side.
Be careful about what resort you pick. They cater to different markets (honeymoon, surfers, families with hoards of screaming kids, gay & lesbian, singles, backpackers, celebrities & the super rich, diving, corporate, large groups, spring-breakers, and more). Also note that just because its a "dive" resort, this doesn't mean its in the best diving area. Some small resorts are family owned, so you're going to get in the middle of "the family" whether you like it or not. Some of the resorts are simple breathtaking, with gourmet food all the way. Others are very rough and rugged.
Take what your travel agent tells you with a grain of salt. Talk to more than one. They sell the resorts they are affiliated with and collect the commission from and I found that I got the most incorrect info from agents (sorry to you agents, but I'm just telling it like I experienced it, and still do today).
I had heard a lot of hype about the diving around Matangi, Qamea, Taveuni, and the Somosomo Straight (great white wall), but I was disappointed. Lots of dead coral rubble and soft coral areas were few and far between. There was also a lot of current. What soft coral I found was small and in small patches. Nothing compared to other areas.
Beqa diving was OK, but not outstanding. I wouldn't travel all the way to Fiji to dive only there. I've heard great things about the shark dives they do in that area, but never did it myself. This area might be your best bet for both surfing and diving.
Nadi, Suva, Coral Coast, etc. - not great for diving. Lots of big resorts, some "Club Med" style.
Yasawas are great if you want the most beautiful white sandy beaches, but the diving isn't that great. Resorts range from super five-star to backpacker.
Mamanucas is pretty good for hard coral, but soft coral is not as abundant. I did see much nicer hard coral gardens than I expected after the coral bleaching they had some years back. Not all resorts have good dive ops or staff that knows what they are doing.
Kadavu - resorts are not terribly luxurious, food and diving is mediocre.
Lau - some nice diving, but hard to get to.
The BEST diving in Fiji is found by live-aboard or long boat trips. The Vatu-i-ra pass in the Bligh Channel is by far the best place to dive in Fiji. You can get there by live-aboard or from Wananavu Resort with Kai Viti Divers. I suspect that the dive op at Koro Sun Resort on Vanua Levu might also go there. Another area that is amazing is around Namena. Nai'a took us to a place they called "Kansas", which was amazing. Both areas have a lot of current, but seeing soft coral gardens as far as the eye can see is worth it. Koro Sun Resort is much nicer than Wananavu, but getting there by plane adds its challenges. Both the dive ops at Koro Sun and Kai Viti were the best in Fiji. Castaway Island also has nice boats and a top notch operation, but has lots of kids too and is kind of pricey.
My advice you be to talk to a lot of people who have been to different areas and ask specifically for what you want when you talk to a travel agent.
No matter where you go in Fiji, you are going to LOVE the people. The minute you step food on their island, you become part of the family and leaving is very, very hard. They sing Isa Lei to you when you leave and definitely capture a piece of your heart. Your trip to Fiji will leave you with memories that will last a lifetime.
Good luck in your decision.