After reading about, Reef Etiquette, from the post “the secrets of good snorkeling in Hawaii, and ReefTeach,” I contacted the Program Services Coordinator at the Kohala Center (www.kohalacenter.org) about sunscreens. She replied that reef scientists are still looking at the effect of sunscreen on corals. Any kind of added chemical in the water could prove a threat to reef organisms. One school of thought is that sunscreen can form slicks on the surface of the water and prevent sunlight from reaching the coral - this could be harmful as corals needs sunlight in order to obtain energy from the symbiotic algae that lives in its tissue.
She said that there are a number of organic, chemical free sunscreens you can buy. Some of the health food stores on the island have cosmetic brands that are organic or better for the environment than your standard sunscreens. The health food store in Waimea for example has the following brands: Alba, Jason, Natures Way, Aubrey.
Here is a link to Aubrey Organics sunscreen online: http://www.aubrey-organics.com/spec_prods/sun_care_0506.cfm
Here is a link to Jason sunscreen products: http://www.jason-natural.com/products/sunbrellas.php