Unless you are very experienced divers, you probably won't want to be diving Rainbow Reef as there is quite a bit of current to contend with there. The Manamuca & Yasawa Island chains offer terrific holiday diving and the areas are well supplied with soft corals. They will also be less rainy than the northern and eastern islands particularly at that time of year.
Most of the budget resorts in Fiji will offer access to either in house or local dive operators. Some to consider are Oarsmens Bay, Manta Ray, Barefoot Lodge in the Yasawas or Beachcomber & Bounty in the Manamucas in addtion to those mentioned above.