I have to add to the Extended Horizons recommendations. They have the only 100% biodiesel boat, small groups, complimentary DM to show you all the cool stuff, leave out of Lahaina and go out of their way to teach you about the areas you will be diving and all the species you will see. The Cathedrals are a must-dive. My wife and I dove with them two years ago and were surrounded by multiple pods of Hawaiian Spinner Dolphins on our first dive...it was amazing. We are headed to the Big Island on Saturday to do the Manta dive and back to Maui next week for more dives with Extended Horizons.
Other than that, we shore dive all week. Slaughterhouse, Kapalua Bay, Mala Pier and Makena Landing are our favorites. We always stay in Ka'anapali and rent tanks from Maui Diving Scuba Center. You can buy a 10 tank card for $40 and they don't expire so we never have to worry if we have tanks left on the card because we know we're coming back.
Have a great time!