Both areas are kind of touristy but Kihei hides it better. Lahaina (Kaanapali) is what you Don't want - the big beachfront comdos/resorts dump a lot of people on the beach daily. Even Lahaina itself is a nice place to visit (shops, restaurants, art galleries) but I wouldn't stay there.
Kihei is a lot of condos in one area but north or south is pretty quiet. The north end is "old" Maui but it's a lot of guest houses next to the owners house, on VRBO if interested. All the Kihei boats go to Molokini and are smaller since they leave from the Kihie Ramp nearby. Just south of Kihei is Wailea - although there are some big waterfront resorts you hardly notice - our condo was around the point from one and I didn't notice it for a few days,
Practically Every beach in Kihei/Wailea is a shallow shore dive, Most have good snorkeling also, usually at either end of the bay where the sand ends and the lava takes over,
One more "Old Maui" area is Napili Bay to the north. Lots of smaller, older condos beachfront - at some you step off your patio onto the beach. Not a lot of food options for that and the dive boats you have to drive down to Lahaina - about 20 mins. Good snorkeling along the north point of Napili Bay - we stayed narby once,
Just don't rent anything in Kaanapali or Kahana and you'll do OK. Why you don't want the Kaanapali area:
http://www.luxuryrealestatemaui.com/uploads/Kaanapali-Maui.75.jpg
Although Black Rock - the rounded point seen to the left is both a good snorkel and shallow dive. Tiny Bubbles dives in the area if you need a buddy.