It's been a couple of years, but location-wise, North Kihei is just about the best; pretty much at the crossroads for the Lahaina/Kaanapali resorts, county seat of Kahului/Wailuku, and the south resorts of Kihei/Wailea. The only exceptions are when you want to go to Hana, "upcountry" Kula/Makawao, or Haleakala; to get to those places, you'll have to backtrack to Kahului. (There used to be a shortcut to Ulupalakua Ranch from Kihei, but it's been closed for years; I don't know if it's been reopened or not.) Do try to get in some horseback riding in upcountry; there used to be a stable just past the protea farms and homes, within the first half-mile or so of turning up into Haleakala National Park; there are other stables in Makawao and Olinda.
Good snorkeling can be found at Olowalu which is a drive toward Lahaina. You'll drive past a rocky cliff coastal road which dips to a fairly long sea-level stretch. Olowalu is just past that stretch, at a major bend in the road with a lot of big trees, an old sugar mill, and a line of trinket shops and such housed in an old building. The only caution I will add is that the area has a sharky reputation (possibly because the area is so healthy and can support apex predators) -- tigers in particular; what the eggheads call "large individuals"; there have been fatalities.
Mike Severns runs one of the two best dive ops out of Maalea Harbor (they might also launch out of the Kihei Boat ramp in the middle of South Kihei). Ed Robinson runs the other, out of the same places. Both are real old-timers and just don't get bad reviews. They don't run "cattle boats" and prefer that you be reasonably current in your skills; bring your logbook just in case -- I don't think they'll take casual snorkelers. I remember that B&B Scuba in the Azeka Marketplace (which straddles the main road, B&B is in the "makai" or seaward half) does tech-stuff and supplies tri-mix. Lahaina Divers has been around a fairly long time but it's been many years since I went with them.
The Kihei beaches in front of the condos are pretty much a disaster. Long ago, before the tourist boom, those beaches were gorgeous. However, some good snorkeling and diving can be found beyond the Wailea/Makena resorts; La Perouse Bay and that area is nice. A good idea is to bring your own rinse water; I use a SunShower. There're a number of other good spots along the whole southern coast but they' usually require humping over steep lava fields in lonely places; these are used mainly by freedivers/spearfishermen.
I never had much luck from the shore up in the Kaanapali/Kapalua area. The winds and current were always moody and I never got a break.
