Poipu Beach area has moderate to good snorkeling, however, we were more impressed with the snorkeling at the Lawaii Beach area (this is right near The Beach House restaurant and across the street from the Lawaii Beach resort). The beach does not look very impressive, however, the snorkeling is great. We also did Snuba in this area and had a great experience.
Ke'e Beach (farthest north you can go) was an exceptionally beautiful beach that looked like it had great snorkeling, however, we could have kicked ourselves as we forgot our snorkel gear and you can't rent it at the beach. Get there early! There is limited parking and it does get busy, but it's definitely worth it.