Dave,
Since you are going to be at the Naviti, if you want to go on a more professional (and probably more successful) charter than resorts often provide the ones I would recommend are at Mango Bay and Pacific Harbour.
Mango Bay is closer and featured in August 2006 fishing world, and though I have a standing invite to get out there I haven't quite made it yet. However, all reports I have heard very very good about popping for GTs etc. I am planning to get there sometime next year.
Alternatively, you could go down to Pacific Harbour to Xtasea Charters. It is a longer drive up the road towards Suva. Their new 40ft white cap Wai Tadra placed 2nd in the Pac Harbour International Tournament earlier this year and they are in the spring 2006 issue of sport fishing marine and trailer boats.
To be truthful, there is a little personal interest as my uncle owns one of the smaller boats that is available for charter under Xtasea, Yatu, a marlin broadbill cat. They are not necessarily better than Mango Bay as I just know more about them.
Other than that, enjoy your trip and PM me if you need more info.
Brendan