I dove in Molokai in November 2007 with Molokai Fish & Dive and am pretty sure they're the only dive operation there.
The diving was pretty good, though not as good as Kona on the Big Island. The shop and crew staff were all polite, professional and enthusiastic. I rented well used, though not abused equipment, including regs, bc, wetsuit, fins, mask and even a dive computer was provided.
All that said, however, Molokai was not a particularly interesting topside location. There's very little to do and any restaurants that do exist, close around sunset. Expect a very slow pace.
If you do end up going, then I highly recommend hiking into Kalaupapa and doing the tour. If you're physically fit enough to dive, then you can do the hike; don't bother with the mules. But, be sure to bring some close-toed hiking or tennis shoes. Lots of manure to contend with! (The hike may start at greater than 1,000 feet, so be sure to consider this when planning your dives)