I second Maui. Esp. with 4 divers as boat diving can get really expensive fast at 2/$100-130 daily. If you're staying in West Maui, Lahaina Divers has two big easy to board 46' Newton dive boats. They run both Molokini Crater and the Cathedrals off Lanai - the signature dives on Maui. Both suitable for your skill level. Extended Horizons has a smaller boat and only goes to Lanai. Both are good outfits.
So for shore diving - in West Maui, get tanks at one of the Maui Dive shops - they have a 10/$55 fill card. Lahaina Divers or Extended Horizons also rents tanks and does guided shore diving locally. A few good sites - Black Rock, Old Airport Beach, Kapalua Bay/Beach. All are simple entry shallow dives where you'll see turtles and possibly some sharks. If you want a guide, Lahaina Divers does that. So does Tiny Bubbles. Here's a good starting point for dive sites.
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands
in South Maui boat operators only go to Molokini and return to dive the coast later in the day. They have to leave early to beat the tradewinds - some as early as 5:30-6:00AM. Boats are smaller since they launch from the Kihei ramp daily. So there's less divers per trip typically. I recommend B&B Scuba, Ed Robinson's or Mike Severns but there's many other options. Severn's crew are all naturalists so you learn more.
There's more shore diving in the Wailea/Kihei area including many of the beaches. Good dives there are Makena Landing, Ulua, Polo Beach, Wailea Beach and possibly the Kamaole's (more crowded with tourists) Some of the sites have very limited parking so go early. In the Kihei area, guides include Shaka Divers (Doug) or Maui Dreams Dive Co. You can get tanks at MDDC also - or B&B Scuba nearby.
There's also the boats out of Ma'alea, Maui Dive Shop keeps one there as does Maui Diamond. There's no real advantage unless you're staying in the condos there. MDS feeds divers from all their shops so it can be more crowded - still 8 divers/DM but the
one time we dove with them, we were the 4th group off. Maui Diamond takes their own bookings plus several local shops who don't own boats.
Not sure what you read about the vis but I've seen 200' horizontal off Molokini in summer. About the same off West Lanai on a good day. About the only place there's drift diving is either the Molokini Back wall or West Lanai past the lighthouse. With 20 dives no one will take you to either. I have never done a shore dive where prevailing currents changed our plans. You will need to tow a flag at least until you can tie it off near where you're diving.
It's not the Manta night dive but Manta's are somewhat reliably seen at a cleaning station off Umekahame (sp) Beach in West Maui during the day. Some ops do it as a boat dive. The Lahaina operators do their Discovery dives/Cert. dives in the afternoon along the west coast. So those are shallower, easier dives. Lots of turtles, some of the operators call West Maui generically "Turtle Town". At the Carpathian (slightly deeper) they're waiting for you. If you night dive, an excellent one is Mala Pier with Lahaina Divers. Their boat gets you out there much easier than the legal entry - I wouldn't want to try it at night - too much standing coral.
---------- Post added May 7th, 2014 at 11:04 PM ----------
The big island is the only island with a liveaboard. Does that mean anything - I don't know but is is reality.
I think it means the boat owner lives there...
Although due to distance, it's probably the only island where it makes sense. Aggressor hits some of the very far southern sites, the Kona operators are very reluctant to go that far south due to time/fuel concerns. Also they dive there all year since the island shelters that side. As it is, they call it the cellphone LOB since you're never out of range.
Off Maui they'd be competing with local ops at the 3 nearby islands.
I could make a case for Kauai - leave from Nawiliwili, go north and dive a couple days. Then on to Ni'ihau and back along the Napali Coast. But that would be seasonal, in winter it's too rough even for an Aggressor class boat - the 45' day boats don't try it after Sept. So they'd have to run two itineraries and compete with local ops on the south side during the winter months. No one dives north Kauai in the winter due to big waves and rough conditions.
I wish somebody would get permits and do trips into the Marine Sanctuary. IDK if that's even possible unless you're a scientist or a Cousteau. I'd pay big money for that. Check out Voyage to Kure' on pbs.com.