Give
Lahaina Divers a call, they are skilled, friendly and professional. Their DM/diver ratio is excellent, their boats are never overcrowded, and all of their DMs are knowledgable about the local sealife population, including endemic species, names and background information (if that is your thing).
I'd recomend hitting the
Maui Ocean Center (its an aquarium) as well, if only to tour it. They allow diving in their shark tank, but its a bit pricey.
Molokini Crater and Cathedrals are really great dives, I'd make sure you get to those.
Personal Experience with Lahaina Divers:
We had a DM who actually dived with Jaques Couteau (I can't spell), and he could have been a marine biologist based on the information he had on just about every subject relating to ocean life.
We spotted a 30 foot whale shark on our way to a dive, and the captain and crew got us all ready to snorkle with it quickly. We pursued the shark (without being pesky) for a while, but unfortunately it went deep on us before we could get in the water. Personally, I found their ability to adapt the plan readily and then switch gears back without limiting our dive time impressive.