Im sorry if Im a little too late but Im surprised that nobody has suggested the Yasawa island chain. We've spent a 3 week stopover here en route to the GBR 4-5 yrs ago. I must admit dive shops are very limited in this area and we mostly free dived to 30-40ft during that time. The only real diving we've made was organized by Waya resort. The other options are the budget busting Castaway island and the rest which hosts the "blue lagoon" movie scene where it caters to celebrities!
Most of our stays were with the villagers (sharing the rooms with our hosts), no electricity, no radios, nothing! We got a cultural immersion by living with them. Women traditionally caught the Octopus but was invited by the elders to learn from them. We also spent quite a lot of time spear fishing ("Raba" the traditional way of spear fishing which the spears didnt have a retrieval mechanism!-meaning if Id miss, which I did 99.999%, I had to dive to depths of 30-40ft just to retrieve the spear-the gun was made out of carved wood which had a rubber or tire material attached to it!)
When we did all these water activities with the villagers (including a real "kava" ceremony), we saw excellent potential for shore diving, snorkelling or free diving just meters away from the shore! The corals and marine life was almost 100% pristine condition.
Im also suggesting that you can easily hop on the yasawa flyer and island hop all the way to the last island. You might want to avoid the party islands of the Mamanucas island groups (first leg of the yasawa chain). The real yasawa starts at Waya and ends all the way to the north. The cave up north has incredible swim throughs.
good luck!
