Should You Go To Maui To Scuba Dive ?

No, if you are coming solely for the diving.
I do not consider Maui diving to be “world class”. If one takes a trip to Maui for any variety of reasons then certainly include some scuba diving.  I have dove in Grand Cayman, Roatan, Cozumel, Fiji, Palau, Kosrae, Pohnpei, Yap, Guam, Bali, various parts of the Philippines, Okinawa, Japan and Koh Tao, Thailand. Maui takes a "back seat" compared to most of these locations.
Maui scuba diving is expensive. A two-tank boat dive costs approximately $120.00. One tank guided shore dives approximately $60. Shore diving is hit and miss as it is affected by ocean swells and overall visibility.
I have been diving Maui since 1993 and have noticed a substantial decline in the tropical fish on the near-shore reefs
Many, if not most, of the underwater photos on my website were taken on small "patch reefs" in the 60 to 80 foot depth range. These areas are not visited by dive boats and are not accessible as shore dives.

Hyperbaric Chamber

Maui has a hyperbaric chamber. It is located in the Wailuku area at 2065 Main Street, Suite 112. The phone number is 808-249-0393. They accept DAN Insurance..

Molokini Crater

Molokini Crater is approximately four miles off the South Maui Coastline in the Makena area and is a Marine Life Conservation District. Molokini offers the best scuba diving on Maui. The waters are clear almost every day of the year. There are several dive and snorkel boats that visit Molokini on a daily basis. Unfortunately, that places as many as 2000 or more people in the water, most of them snorkelers that are inside the crater. The larger snorkel tour boats tie off to mooring pins along the curve of the inside wall.
The scuba dive boats tie off to mooring buoys inside the crater and/or drift with their divers after putting them in the water. The currents vary from day to day and can be strong along the outside of the crater. The diving is good inside the crater and depths can reach over 100 feet. In my opinion the best diving at Molokini is outside the crater at "Reef's End" and the "Back Wall." Depths can reach over 300 feet so one must closely monitor their depth gauge. 

A: Inside the crater. This area will be occupied by many snorkel boats along the wall as well as dive boats at the mooring buoys which would be mostly located at the area known as Reef's End (B.) The diving is good with depths up to 100 feet. This is the most crowded area of Molokini Crater.
B: "Reef's End"  Dives begin inside the crater and continue outside along the reef at varying depths with a return to the boat by crossing over the top of the reef which can be as shallow as 6 feet . Another option is to start inside the crater, scuba along the outside reef, and continue on around the Back Wall getting picked up by the boat wherever you surface. This is one of, if not the best, dives at Molokini. Currents can be strong on the open ocean side of the reef. I prefer to do this dive by continuing around to the Back Wall but the individual dive operators have their own routines.
C: "Back Wall" Many boat operators will put their divers in at points along this area and retrieve them upon surfacing. This is also an excellent dive and an opportunity to do a wall dive if you have never done so. The depth along here is over 300 feet with crystal clear waters. The S/W corner area of the Back Wall quite often has a surge which can move you up and down like an elevator by as much as 10 feet, depending on conditions. Caution is advised here.
D: "Shark Ledges" This dive is inside the crater. Depths can reach 160 feet.  Currents can be very strong at the point where the inside and outside of the crater meet, and can pull you out and around towards the Back Wall. You can feel the current building so as to avoid it if the dive profile is to remain inside the crater.

Boat Diving

Boats operating out of the Kihei Boat Harbor (boat ramp) are closest to Molokini Crater with only a 15 to 20 minute boat ride. The larger boats that leave from Ma'alaea Harbor take about 45 minutes to get there. Boats out of the Lahaina area take even longer. The Trade Winds come up around noon almost daily and this makes for a rough ride back, especially to the Lahaina area. I recommend taking dive boats out of the Kihei Boat Harbor and making the drive to it if you are staying on the upper West side. 
Does snorkeling and scuba diving mix? Not in my opinion when it comes to charter boats here on Maui. For the most part the scuba divers don't get the best diving and the snorkelers don't get the best snorkeling. Snorkeling is usually best in shallow water and scuba diving varies depending on the dive site. One exception to mixing both on the same boat is for a family/group who want to stay together and some of them don't scuba dive. If you want to snorkel, take a snorkel boat. If you want to scuba dive, take a dive boat.
A couple of boat operators out of the Kihei Boat Harbor in South Maui are Mike Severns Diving and B&B Scuba

Guaranteed Boat Dive Sites

Molokini Crater is almost always 1 of the 2 dive sites when booking a 2 tank boat dive.
Do not be misled into believing you are guaranteed a "Back Wall" dive. Operators choose dive sites based on the experience of the divers on board the boat on the dive day. If any of the divers object, or are not qualified to dive the Back Wall, then it won't happen.
As of June 2005, Maui Dive Shop is the only operator I am aware of that does a dedicated one tank Back Wall dive from their Kihei Harbor based boat three days a week. They give you a discount if you do the 3 tank dive package but be advised that involves two boat trips. First, a boat trip to Molokini for the Back Wall dive and a return trip to the Kihei Boat Ramp. Then a second trip to Molokini for 1 of 2 dives with the two tank trip customers. I am not recommending them as a dive operator other than for the specific "Back Wall" dive .

Shore Diving

There is plenty of accessible shore diving on Maui's West Coast (leeward side). The West Coast is divided up between the South end being the Kihei, Wailea and Makena areas and the West Side being the Lahaina, Kaanapali and Kapalua areas, amongst some others. The diving conditions will vary depending if there are ocean swells. You have to check for that on a day to day basis. The best shore diving guide for Maui is "The Divers' Guide to Maui" by Chuck Thorne and can be purchased from him online for under $15.00.

Shaka Divers offers guided day/night shore dives, private dives, scooter dives, certifications and more.

Rental Gear

One of the least expensive shops to rent dive gear is Maui Dreams Dive Co. in Kihei. They are located in the ground floor shops of the Island Surf Condominium building at 1993 S. Kihei Rd.

Ni'ihau Diving

Ni'ihau, which is accessible by boat from the island of Kauai, offers the most pristine diving in Hawaii with the exception of the Northwest Hawaiian Islands which are not open to the general public. Bubbles Below and Dive Kauai offer trips to Ni'ihau from May thru September. This is advanced diving and ocean conditions on the return trip can be very rough. The trip is an all day affair. Check the respective dive operators web sites for details.