Hi,
Each of the islands is very different, both in what is on land and what is underwater.
Let's start with the north...
Kauai is the oldest of the four main islands. I think the best diving is on the Poipu side, but some folks like the Princeville side. Since Kauai is oldest of the four I will cover, it has the most "muck diving". If you want to see Frogfish and Turtles...lots and lots of Turtles..it is the place to go. I have not been there in several years, but if Bubbles Below is still in business, they are great.
Oahu is the home of Honolulu and Waikiki and, as such, is probably the major tourist mecca. I am not fond of diving Oahu and, so, will leave that recommendation to others. If you are looking for world class surfing and shopping and wonderful history, Oahu is the place to go.
Maui is both a major site for scuba, beaches, surfing, touring and golf. There is some fantastic diving off of Molokini, especially on the backside. It requires good bottom times and it is worth the trip all by itself. It is also a reasonably quick boat ride to Lanai and that is some mighty fine diving.
There are a number of dive outfits that I have gone with and can recommend Ed Robinson--especially for advanced divers--Maui Dive Shop and Lahaina Divers.
While most shops have both morning and afternoon dives, I strongly recommend the morning dives. The tradewinds come up most afternoons and can make a boat ride darn uncomfortable.
The Big Island, Hawaii, has the Kona Coast (I do not recommend the Hilo side since it can have some problematical surges and currents.) and that is just wonderful. One of the nice things about the Big Island is that it is BIG. While not the tourist Mecca of Maui, it has great volcanoes, great sites of historical interest, and darn good diving. I can recommend three dive outfits: Sandwich Isle Divers, Jack's Diving Locker and Pacific Rim Divers. Each has its own flavor and let me just outline them for you. Sandwich Isle has a dive shop and a boat. The boat is nice and only takes 6 divers. The crew is just great. Jack's takes 12 divers and divides them into two groups of 6. They also have an advanced three-tank dive. Jack's has, in my opinion, the best equipment store in town. Pacific Rim Divers just has a boat and it is the cleanest, most organized boat on the Kona Coast. Nice crew, too.
If you are going as a family, I suspect Sandwich Isle could be an excellent choice. Oh, yes, Sandwich Islands was the original name for Hawaii.
I hope this helps.
joewr