Kahana is about 10 minutes north of Lahaina, perhaps 5 minutes north of Kaanapali. Honolua Bay is a bit further north. Somedays it is great diving there during the winter, somedays it's a great surfing spot. Old Airport is on the north side of Kaanapali, very near to Kahana. And Black Rock is in the main resort section of Kaanapali. Both are nice, are close to Kahana and are easy dives, although Black Rock is a pretty long hike through the resort to the beach. Boat diving out of West Maui (Lahaina, Kaanapali, Kahana, etc.) is mostly going over to Lanai, with the boats leaving from either Lahaina harbor, or the Mala Ramp in Lahaina.
Lahaina Divers does run some boats from Lahaina harbor over to Molokini, but I strongly recommend driving over to Kihei and getting a boat from the much closer Kihei boat ramp. It's about an hour drive from Kahana to the Kihei boat ramp (assuming you stay on the highway that bypasses Kihei and don't exit until Keonekai Rd / MM9).
Wailea is the next city south of Kihei, and the Kihei boat ramp is just a few minutes away (it's at the south/Wailea end of Kihei).
There are some pretty nice shore dives in both South Maui (Kihei/Wailea/Makena) and West Maui (Lahaina/Kahana, etc.), so it doesn't make a lot of sense to drive to the other section for a shore dive unless it is part of a trip to see other stuff too.