Bula, There isn't really a place like Cozumel in Fiji as resorts are generally a little isolated with limited transport between them. Pacific Harbour as Boris says above is the closest. There are 2 operators there, Aquatrek and Beqa Adventure Divers who both offer shark dives and coral dives in the Beqa Lagoon. The accommodation options there are, in order of ascending cost, Club Oceanus, Uprising and the Pearl. There are some very down market places there aimed at backpackers and a few private villa rentals, which are are nice option for self catering and great if there is a small group.
Perhaps your best bet might be one of the bigger dive centric resorts, specifically Beqa Lagoon Resort, Volivoli or Wananavu. All these offer a full on dive experience and account for a big chuck of Fiji's diving. There are smaller places with diving as a main feature like Matava and several resorts on Taveuni, both of which require an internal flight to reach - but arguably very worth while.
And of course the 2 live-aboard boats, Naia as mentioned above and Island Dancer II, operated under the Aggressor franchise (the boats name is actually Sera-ni-Wai) Both good, but Naia is the better product for sure. If you have 2 weeks, split it between 2 locations, or possibly 3.
Happy to give more details if you want.
Have a great trip.
Greg