Each of the islands has their own highlights and points of interest for sure. I feel that if the vacation focus in on diving that you would be best to choose from Big Island, Maui or Kauai. Maui is probably the busiest of the three with Kauai being more on the "quiet" side. Each island also offers very professional options for divers of all interest and skill levels. You really are in a win-win situation with choosing in my opinion. Narrow down what your vacation focus is and what other needs and interests you have and I think you'll find the answers and suggestions start flowing in for you. Do you plan to do and self guided shore? Shore or boat guided?
Take a look at the Manta dives on Big Isle out of Kona for sure (which it sounds like you have). Some divers enjoy the Pelagic Magic dives over there as well. WHOA (Wild Hawaiian Ocean Adventures) is a must do trip if you visit Big Isle in my opinion. For Maui you should look at Molokini, Lanai's Cathedral dives, and shore dives "Mala Wharf" and "Makena Landing." On land the Maui Ocean Center is fantastic. I think dive with Pauline and her staff at Mike Severn's is a must do. Kauai research should certainly include Niihau, which in my humble opinion is some of the very best diving in the islands. Koloa Landing is a fantastic shore dive here with endless critters. A trip to the Waimea Canyon is fantastic for hikers (just not after diving).
Potentially the most important thing to be aware of is that in the winter time the northern shores of all the islands are subject to large (sometimes jaw dropping swell). This can eliminate or at least make any diving on northern/northeastern/northwestern shores difficult or impossible. This should factor into the considerations when you are determine where you are staying. Nothing worse than finding out you have to drive an hour or more every day to get to dive.
There's a ton of people here that will help you as you narrow things down. Please feel free to send me a PM on here if you desire. I'd be happy to help with any questions that you may have to help make sure you have a fantastic trip to Hawaii.
Cheers
A