The Solomon Islands in the Western Pacific are not particularly well known in tourist terms because access is really only from Australia. Fewer divers get the chance to venture there than would like to. It's a shame as the Solomons recently became included in the Coral Triangle.
Once you descend underwater you can see why: marine diversity is high, there are coral covered reefs and steep walls, secretive critters and schools of pelagics. Small cave systems lead beneath the coast so you can swim through them and then surface inside shallow mangroves where archerfish hover amongst the roots.
This country is also important historically as it was the location of some of the longest battles of World War II. Around many islands, there are remnants littering the sea bed and some very interesting wreck dives. What's particularly good about them is that, outside of Guadalcanal, they're not all that deep so you can spend a decent amount of time exploring. There's even a fascinating munitions dump with trucks, barges and stray ammunition littering a calm bay.