I second Severns for the reasons above. They do leave early (6AMish) though - all the Kihei boats do since they launch from the ramp and like to be back b4 10am to beat the tradewinds b4 2nd dives along the coast. Reef's End is a good dive on the right point, my buddy likes Shark Condos also but I didn't dive it. Kind of a good dive site summary is:
Molokini Crater Scuba Diving | Molokini Maui Dive Sites Although they don't mention it, a insta-buddy and I found reef sharks sleeping in the sand at Tako Flats also. As we got close they'd move 100 yds. then settle down again. We only chased them once...lol
An interesting second dive - if they'll do it - is Umekahame Beach off West Maui. It's really hit/miss though since it's a shallow water Manta cleaning station. If they're there, it's pretty amazing, if not it's pretty average or even slightly boring - they might suggest one of the turtle sites instead.
While there's shore dives in Kihei, IMO there's better shore dives in Wailea to the south. The Kihei beach dive sites (Kamaole's etc.) see a lot of beachgoers from the hundreds(thousands?) of condos just across the street behind them.
Everyone has their favorite but mine is Wailea Beach - esp. the south point. I've seen a family of Cornetfish out there, big Puffers, lots of tangs and surgeonfish of all sorts - including Naso's and Powder Blues - we also saw some that looked like Powder Blues but were brown/white - a whole school of them. There's also the rare lagoon triggerfish found there. I've only seen one swimming away. Lots of turtles and morays also. Some interesting overhangs in among the jumble of lava on the point as well.
If you plan to shore dive in that area get there early, parking lots will be full b4 9AM. Since there's often resorts on either side - the Grand Wailea also shares Wailea Beach - there's no other options for parking nearby. We've done alright late afternoons also.
You might consider hiring a guide also. Both Doug at Shaka Divers or someone from Maui Dreams Dive Co. are options - there's probably a dozen more.
People dive Hawaii and then report back on how barren it was - esp. compared to the Caribbean. Which it is to some extent but it's often that the fish have learned to hide better. Once a guide shows you they become a lot more apparent. Hawaii in general has one of - if not the largest populations of endemic species in the world. I suspect a lot of them are in the marine reserve but still...
I've seen just about the largest schools of fish in my life off West Lana'i - one DM estimated we saw 5000 Moorish Idols once holding in the current off Sharkfin. Bunches of tangs/butterfly's also. And it's not considered the fishiest site. To get there from Kihei you'll need to drive to Lahaina - about 45mins. The only good thing is the boats leave at an almost civiilized 7AM. I'd go with Extended Horizons.