I've been here a couple of times before. The absolute must see is a Manta Ray Night dive. My wife and I both dive and I'm an underwater photographer - so Hawaii is a great place for visibility and marine life. The nice thing about diving in Hawaii is that all the dive companies are really good - especially when compared to some out of country destinations.
Be prepared to go on boat dives because they are much easier than shore dives - especially in Kona. And it is very weather dependent because the opening to the boat docks can be challenging with high waves - so stay flexible. If they can't safely get out of the harbor, you really don't want to dive in that kind of turbulence anyway.
Save up and be prepared to spend $110 to $140 per 2 tank dive (it's more for the night dive with the Manta Rays).
Oh and get really good at clearing your ears and start that process on land before you hit the water. Also, if you get a little boat sick - Bonine is the best because it works and it won't make you drowsy. Drowsy diving is dangerous.
Oh and make sure that you don't freak out if you see a shark. Sharks are awesome and they are like cats - some are like house cats, some are cougars, some like tigers, and some are like lions. None of them want to eat humans - they only go after things that look like seals. Thankfully seals don't have metal tanks strapped to their backs - so sharks, especially those in Hawaii don't go after divers.
But that means - do not swim alone in the morning or at dusk and especially not near the opening of a river or stream. And DO NOT SWIM WITH SEALS - seriously - that makes you the slowest one in the group.
I recommend putting on a dive skin that is multicolored over your bathing suit etc - check them out at DiveGoddess.com. They are brightly colored lycra skins that stretch like crazy and you definitely will not look like a seal in the water with these on.
Have fun! And see you there.