My wife and I were there the week after Thanksgiving last year and I had a blast fly fishing. We stayed at Kona Village and I fished the beach there several mornings getting an assortment of fish, the coolest being blue trevally. There were huge bones on the beach but they were deep, I could see them snorkeling but never figured out how to actually catch them and I tried. Milkfish abound too but again, deep running. There are bonefish, cuda, and some giant trevally in the old fish ponds just north of the airport, I had a huge GT slam the fly at my feet in there and scared me half to death. Google around a bit, there are a few good pages out there on Big Island fly fishing that cover the beach areas pretty well, that was my starting point.