Maui is a great place for diving, whether you're a beginner or advanced diver. If you want to boat dive and want to stay close to Kaanapali, I'd recommend Lahaina Divers to you. They are very professional and do a great job with beginners. Just tell them you're newly certified and the DM's will give you extra attention. You definitely want to go to Lanai and dive the Cathedrals...it's an experience you'll never forget. Extended Horizons also goes out of Lahaina, but I've never gone with them since they're quite a bit more expensive and run a smaller boat. I prefer the big 46' Newton dive boats that Lahaina Divers runs to Lanai because the crossing can be pretty rough (especially coming back at 11:30) and I'm prone to seasickness problems. But that's just a personal preference, I have nothing negative to say about Extended Horizons.
That said, I'd really recommend if you want to dive Molokini Crater (which you really should), it's much better to drive down to Kihei and catch a boat down there. It's about a 40 minute drive early in the morning, but with the boat ride being only 15 minutes to Molokini, it's well worth it. B&B is a highly recommended operator in Kihei and everyone says they do an excellent job with beginners as well as advanced divers (just ask Al or Tim!).
For shore diving, a really great (and very easy) dive is Black Rock at the other end of Kaanapali Beach from the Hyatt (in front of the Sheraton). Max depth is 30 feet or so and navigation is a snap. You simply keep the wall to your right on the way out and to your left on the way back. You can either rent tanks from the kiosk at the Sheraton (they are only AL64s and expensive but convenient) or you can get AL80s from Maui Dive Shop at Whaler's Village. You definitely don't need a guide to dive Black Rock, that's for sure. Actually, right in front of the Hyatt is a decent reef dive called "Black Ball", but I've heard the swim out can be challenging (I've only done the dive from a boat). I'm sure the Hyatt could arrange guided shore dives for you if you want...I know there's a kiosk next door at the Marriott that does shore dives right in front of the resort.
Kidspot is an excellent resource and if he's around, he's usually more than willing to meet other board members for a dive. Have him take you to Mala Wharf...great dive!
As far as sharks in Hawaii are concerned, I don't really think you have anything to worry about. We've seen both White Tips and Nurse sharks and neither cares much about us when we're diving. We leave them alone and they leave us alone.
As a side note and not specifically related to diving in Hawaii, my husband and I did a shark feeding dive in the Bahamas a couple of years ago. We were in 50' of water, overweighted, kneeling on the bottom while 3 guys in chainmail fed the sharks. There were approximately 30-40 Black Tip Reef sharks ranging in size from 3' to as long as 6'. As we were watching, the sharks were bumping us with their fins, one ran over the top of my head so close his stomach rubbed my head, and another almost knocked my husband's mask off. They would swim between us (and we were only 6" apart) and knocked one guy completely over in the sand. At NO time were we anything but other "critters" in the water. They didn't see us as food for many reasons, the biggest ones being that we didn't act like food (injured and/or erratic), and frankly, we're too big to be considered prey they would hunt. Were we 100% safe from a bite? Heck no, you're dealing with wild animals and can never consider yourself 100% safe, but frankly we were safer from being bitten by a shark than we were from being stung by fire coral in the Caribbean.
