Ok not a lot of spearfishing going on in Fiji. Just about every reef or beach is traditionally owned, there's a local village that has the rights to that place. You need to talk to the chief of the local village about fishing or else it's like breaking into someones property and stealing stuff. It's not hard as Fijians are the friendliest people on earth and the chief will be happy for you to visit the village and talk to him about what you want to do. Take some powdered kava as a gift, like about a kilo, and things will go off smoothly. Dress respectfully, don't turn up in your speedos.
You will find heaps of coral fish but most arent good eating. Look for tuna type fish, like trevally. Coral trout should be your best bet but you need to be fit. Spearfishing on scuba is very bad form - you need to be freediving so give up the ciggies and start exercising. You need to be quite cardiovascularly fit as the big fish are in deeper water like 30 to 40 ft. You will have a lot of fun in Fiji and if you dont get any fish dont panic there's plenty of fish in the wet market at Savusavu. Oh and don't take female crays (lobster) with eggs, bad form again. Good luck.
