We have been diving Kona for years and love it! The vis always seems to be 100+ feet during the day and the sea life is abundant. One nice thing about Kona (if you are not a sailor) is that is it is protected from the trade winds. That means some dive shops have later starts and you can have a more leisurely morning compared to, say, Maui.
We have tried a number of shops and prefer Sandwich Isle Divers. They take only 6 divers at a time and usually there are 2 DM's + the boat captain: talk about personal attention! And the price of their dives is the same as the other shops. One of their DM's, Walt, is the amongst the best fish finders we have ever dived with. Want to see a Titan Scorpionfish? Just ask Walt!
One other nice thing about Kona is that the geography features a very steep drop-off and, so, you get out to most dive sites quickly. In addition, there is precious little current on most days, so the diving is easy. However, if you like drift dives, Kona may not be the place for you.
The Big Island is our favorite in the Hawaiin Archepelago--lots to see under the sea and lots to see on the land. It is not as "touristy" as Maui, so do not expect the same kind of night life: but, heck, you are there to dive!
Scorpionfish--Only Its Mother Could Love This Fish!