Where are you staying? There are shops/boats in the Lahaina area or shops/boats in the Kihei area. It's about a 45min drive between them and the Kihei boats leave around 6am to get over to Molokini b4 the tradewinds pickup (smaller boats)
Maui Dreams Dive Co. would be a good choice for the g/f in Kihei - they do their referrals off a beach nearby. They're mostly a shore dive operation although they have a boat in Ma'alea about 10mins north of Kihei. All the south Maui boats either go to Molokini or dive along the coast. The Lahaina boats go to Lanai except Lahaina Divers sends a boat down to Molokini once or twice a week. All the Kihei-based boats leave from the Kihei boat ramp sort of on the north end of town. Maui Dive Shop or MDDC's boat leave from Ma'alea about 10mins drive north. The Lahaina boats all leave form Lahaina harbor.
There's more shore dives closer together in South Maui. Many of the beaches are shore dives, Polo, Ulua, Wailea, Makena Landing, 5 Caves etc. Most of these are in Wailea which is south of Kihei.
There's not really a good shore dive in Lahaina but Sheraton Black Rock in Ka'anapali would be an easy dive for your gf and kind of fun - snorkelers overhead.
Old Airport Beach is an easy entry dive a few minutes north but it's not the best near shore. Napili Bay further north is decent, past it is another shallow dive at Kapalua - which is also one of the nicest beaches on Maui. Not always but at most sites the more interesting stuff is off the points since they're often volcanic. Tiny Bubbles is based in Ka'anapali and all they do is shore dive. Extended Horizons does in the afternoons also. Mala Pier is a good dive, it's shallow, the legal entry is over some rocks but it's not bad. If a DLNR agent is in the parking lot, don't go down the ramp - it's a ticket.
Here's a list:
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands
Lahaina Divers has 2 big boats and they usually go to Lanai. The Cathedrals are the signature dives there - there's 2 and they're somewhat overhead obstructed but have a lot of easy "outs"
The farther west you dive Lanai the more challenging - there's some fast drift dives. We like Sharkfin, inside the bay is pretty benign with octopus in the wall, around the outside is ripping usually but there's 1000's of fish holding there.
Molokini is pretty benign, it's a sunken volcano that calms the water inside. The corners have some deeper dives and the Backwall is often a live drop drift dive - the bottom is about 300'. Almost always see sharks there - we've even seen some white tips in the sand flats on the protected side. Also one of the deeper dives is Shark Condos.. South Maui shore has a few WWII wrecks - some of the operators do them as a 2nd dive. No preference but Mike Severns runs a nice trip, smaller boat and they're all naturalists. B&B Scuba also and I believe they shore dive afternoons. You can get tanks from them or MDDC - or the main Maui Dive Shop is in Kihei also. MDS has several convenient locations along the entire west coast - one in Kahana Gateway mall is handy for the northern west side dives as you can pull right in off the main road.
Besides Lahaina Divers - who have two big Newton Dive boats (for the gf) there's also Extended Horizons and Hawaiian Rafting Adventures. Once or twice I've seen people coming back on EH's boat - they're often wet. HRA runs a hard bottom RIB - I watched it get tossed around pretty good off Lanai once.
One thing to be aware of is there's several booking agencies that will book you on another shops boat but don't clearly indicate it on their website. It's the same boat so why use a middleman.
apparently I've been to Maui Too many tines...lol
although not recently