Hawaiʻi was the first U.S. state to ban two sunscreen chemicals that harm coral, and the rule applies on Kauaʻi just like the rest of the state. The short version: leave the oxybenzone and octinoxate at home, and pack a mineral sunscreen instead. Here is what the law actually says and how to pick a bottle that is fine to use.
Hawaiʻi Act 104 was signed in 2018 and took effect January 1, 2021. It bans the sale, offer of sale, and distribution of any sunscreen containing oxybenzone or octinoxate (or both) anywhere in the state without a prescription. Both chemicals are linked to coral bleaching and damage to marine life. The law targets sellers, so stores on Kauaʻi cannot stock these sunscreens, but the spirit of it is simple: do not put those chemicals on your skin before you get in the water.
Flip the bottle over and look at the active ingredients. If you see 'oxybenzone' or 'octinoxate' (octinoxate is sometimes listed as ethylhexyl methoxycinnamate), it is the kind the law is about. A genuinely reef-friendly option lists only non-nano zinc oxide or titanium dioxide as the active ingredient. These are mineral sunscreens that sit on top of your skin rather than soaking in. Be a little skeptical of the words 'reef safe' on the front of the bottle, since that phrase is not regulated. The ingredient list is what matters.
Bring a mineral (zinc oxide or titanium dioxide) sunscreen from home. You can also buy mineral sunscreen at most stores and pharmacies on island, since the banned chemicals are not supposed to be sold here, though it costs more than mainland prices and enforcement is inconsistent, so still read the label. Mineral formulas go on a little white, so rub them in well. A rash guard or sun shirt and a wide-brim hat do a lot of the work for you and mean less sunscreen overall, which is better for you and the reef.
Kauaʻi's reefs are the reason the snorkeling and the sea turtles are so good. Sunscreen chemicals wash off swimmers and concentrate in shallow, calm bays, which are exactly the spots people most like to snorkel. Choosing mineral sunscreen is one of the easiest things a visitor can do to leave the water as healthy as you found it.