I've always used and really liked Fathom FIve Divers in Poipu, both for shore diving (Koloa Landing in Poipu and Tunnels Beach on the north shore) and boat diving out of Poipu. For boat dive sites, let the Captain decide what is best for the day's conditions when doing a local dive. However, be sure to book a trip out to Ni'ihau ("The Forbidden Island"). That island is privately owned, has never had any intensive agriculture or industry and is surrounded by supremely clear water, jaw dropping underwater topography and critters who don't see divers very often.