We dive Maui every year, or twice every year. Both Molokini and Lanai are great dives, but very different. The backside of Molokini is one of my favorite places to dive everywhere - so much life to see in all the nooks and crannies, and then you turn around and look out into the blue and never know what you are going to see. But it's a more advanced dive than the others. Inside Molokini crater can either be wonderful or just OK, depending on conditions, where you dive, your dive guide and dive profile. The norm is early dive at Molokini, then second dive closer to shore by Wailea or Makena. Again, close to shore is luck of the draw whether conditions are spectacular or just good. Depending on the dive operator, you will typically be back at the Kihei boat ramp between 10 AM to noon. Lots of really good dive operators; my favorites are Mike Severns and Scuba Shack. The near shore diving isn't usually as good as offshore
Lanai is totally different and noted for the cathedrals - visually dramatic and stunning. But it's a longer boat ride going and coming, and unless you're on nitrox you can be more limited on bottom time due to depths. Extended Horizons does a great trip; we've dived with Lahaina Divers to Molokini and they were excellent as well
Depending on your skill level and interest, you might want to check out Lahaina Divers trip to Molokai to see the hammerheads. It only runs a few days a week and is advanced, but Molokai underwater is gorgeous underwater before you head out into the deeper water looking for hammerheads
All the dives are usually two tanks, and like everyone has said, the boats leave early. Plan on arriving at the boat at 6 AM or so
One day isn't enough to experience Maui diving. My suggestion is to blow off your non-diving friends in the mornings so you can dive, and then meet them starting at lunch near where you depart for the rest of the day's activities