Kauai
Been living in the islands for about 10 years now and I'll never get tired of my trips to Kauai. Every year we take a family trip there to do some hiking, some snorkeling but most of all we fly over for the scuba diving! The kids love Koloa Landing and all the things they see there.
From 3 years back, we (the adults) have been taking a separate day trip to Niihau. I gotta say, THAT island is insane. We have been using the same dive shop for quite a few years now (Seasport Divers) for our diving in Kauai and also for the Niihau trips since they are the most knowledgable and awesome staff. I love how kind they are to my wife and kids, how well educated their instructors are and the fact that the boat is well set up for underwater photographers (like me!). They treat us as family and that makes a huge difference as my kids were a bit intimidated by the whole SCUBA diving idea at first. Just go check them out at www.seasportdivers.com and you'll see what I'm taking about. I can't speak highly enough about them.
Overall, scuba diving in Kauai is a great experience, enhanced by the fact that you can take a day trip to Niihau which to me is the best scuba diving in Hawaii by far. After all this years and having tried several diveshops, I've come to the conclusion that Seasport Divers is the way to go.
If you are considering a trip to Kauai, let this be the sign for you to go ahead and purchase the flights and dive trips right away!
PS: Summer is the best season for diving over in Kauai. Weather can get pretty rough in the winter, that's why there are no Niihau trips after October or thereabouts.