West Maui (Lahaina, Kaanapali, Kahana ...) or South Maui (Kihei, Wailea ...)? They are about 45 minutes to an hour apart.
The Kihei boat ramp near the Kihei/Wailea border is the best place to catch a boat to Molokini. There are dive ops that go from Lahaina, but IMO it's a lot better idea to drive to Kihei. Mike Severns, Ed Robinsons and B&B are all popular dive charters.
My favorite dive op in Lahaina is Extended Horizons. Departs from Mala Ramp, usually to Lanai.
There are lots of good, easy shore dives in Maui. Old Airport and Black Rock in W. Maui. Ulua and Makena Landing in S. Maui. Any diveshop can point the way, and all of the many Maui Dive Shop locations have a free booklet detailing dive and snorkel spots.
If you are going to be in Maui the first week of January, then advance reservations are advisable. Otherwise, IMO it's better to get on island, figure out the timing on some of your other activities, and then book a spot. Other than during the super peak periods like Christmas/New Years and spring break, I usually find a spot available even when booking the night before.