Both islands have good diving. The Big Island has the best diving in the state. As for Maui, Molokini Crater is a must. It can get a little crowded. There's an inner portion and an outside wall. The wall is definitely better, but you should be comfortable with buoyancy control and drift diving. It's considered an advanced dive. Some shops make it to the Cathedrals off of Lanai. They're a set of pennicals. It's a fairly long boat ride and it can get choppy. It's also considered an advanced dive. There're numerous closer sites and numerous shore dives.
As for the Big Island, the sites are generally close to shore. Kaloko Arches and Place of Refuge are a couple of my favorites. The Manta Night dive is also very popular. While I enjoyed it, I probably wouldn't do it again. You'll need to be comfortable with night dives. It also gets crowded. There're a lot of great shore dives too. You'll probably be more comftable with a dive master on the shore dives.