If you stay in West Maui your
boat dive choices are Extended Horizons or Lahaina Divers. Both are good and moor their boats in the Lahaina harbor area. EH has a smaller boat. There's a couple of other dive operations in Lahaina or nearby - including all the Maui Dive shops but their boats are in Ma'alea - about a 1/2 hr. drive south. MDS has a nice big Newton but they feed divers from all their locations there so not recommended.
Extended Horizons only dives Lanai and West Maui. The Cathedrals is the signature Lanai dive and there's also some good drift diving off West Lanai. I saw some big schools of Moorish Idols and Tangs riding the currents there once. And some white tips hovering nearby.
LD has the bigger boats and often more divers. Although I've been on their boat once with 6 others - that was nice. They also dive Lanai and on alternate days one boat goes to Molokini - the other signature Maui dive site. In addition they do a Hammerhead dive out to Molokai once a week (Thursday?)
There's some good shore dives in West Maui also - most are beach entries. LD has a list on their website. You can get tanks from them or from the two Maui Dive Shops in the area - at Kahana Gateway Mall or Lahaina Gateway Mall. The shop at Kahana Gateway is convenient if you're staying up in that area - just off the main road.
The signature dive off South Maui is Molokini Crater. In south Maui all the boats leave from the Kihei boat ramp. Typically they're a little smaller since they're rampable. They all leave pretty early in the morning to beat the tradewinds coming back - some as early as 5:30AM. Depending on the dives/operator selected you'll either dive the front side of Molokini or the Backwall. Inside the crater is pretty calm conditions, the backwall might be rougher in winter. Great place to hear whale sound though. Often those dives are live drop/pickup. The back wall is pretty vertical and the bottom is around 400' IIRC.
Those boats often do the second dive off the coast of Maui. Depends on the operator/conditions. Often recommended are B&B Scuba, Mike Severns or Ed Robinsons but theres' others also. There's also a lot of shore diving options in south Maui including some easy beach entries. There's several MDS locations for tanks or B&B or Maui Dreams Dive Co. also rent them.
FYI it's about a 45min. drive between Lahaina and points north and Kihei. Even early in the morning. And if the humbpacks are out, often longer as people slow down/stop to watch them. There's only one main coastal road and you'll be on it.
Mama's Fish House in Paia is worth the drive one night. Maybe make reservations - it's gotten popular.
Welcome to Mama's Fish House Restaurant and Inn - Maui, Hawaii
In West Maui we liked the Napili area. Quieter, some properties are older and there's a few nice beaches. Kahana and Kaanapali are where all the mega-high rise condos/hotels are. And most of the shops/restaurants. Just north of Napili is Kapalua Bay/Beach - one of the nicer ones on Maui. There's a shallow dive there also.
In Kihei many of the condos are across the main street from the beach. Most of those beaches are also shore dives. I like it better in Wailea south of Kihei. There's some big resorts there but the way the coast line runs in some areas you don't even see them. We rented condos that fronted a semi-private beach. All beaches in Maui are public access, this one just isn't as used. If you're interested in that area PM me and I'll find the name of the property. Wailea is more expensive than Kihei generally also.