We've got it down to a bag and a half each for a Maui trip, but if you load your carry-on, you can probably get it down to one. We take everything, and that includes canister lights, which a lot of people, I think, would leave home. You don't need a lot of heavy clothing for Maui!
We have dived with three Molokini boat operations, and all of them are fine. B&B leaves earliest, so they get to Molokini first. It's nice not to be in the water with a bunch of other divers. They tend to attract a lot of new divers, though, and if you are looking to instabuddy, you could have a short dive. Ed Robinson runs a three-tank advanced dive once a week (or at least they used to.) But my favorite is Mike Severns. They tend to serve a more experienced group of divers, and the owner is co-author of one of the marine life guides to Hawaiian Islands. They have critter ID materials on the boat, and try to take you where you will see unusual endemic species.
If you are staying up in the Kaanapali or Lahaini area, it's very worth taking a boat over to Lanai to do the pinnacles and the Cathedrals. Lahaina Divers runs such a boat, but be aware they use 72 cf tanks. I have heard that you can request bigger ones, but I have never put that to the test, nor have I been out with them. (We did our Lanai trip from Kihei.)
Shore diving, if the weather cooperates, can be almost as good as the boat diving (the viz isn't quite as good). In the south end, I like Ulua and Polo beaches. Ulua has a second reef just offshore that is very large and fun to explore. At Polo, enter as far north as you can (and beware of the outrigger canoe!) and you will find a bunch of fingers of rock, covered in some of the healthier coral I've seen off Maui. We've seen nudibranchs, octopuses, and several species of morays there.
In the Lahaina area, my favorite shore dive is the Mala Wharf, even though the legal entry is a PITA. It's a shallow dive, with lots of structure and TONS of life. I've heard there can be current there, but we've never had any. Further up is Honolua Beach, which has a long walk to the shore, but a beautiful wall to swim out along, and if the water is calm, you can swim back over the top of it.
www.alohashoredivers.com is a good resource for Maui shore dives, as is shorediving.com.
Have fun in that warm, clear water!