Where the reef is actually healthy
Not every popular snorkel spot has good coral anymore — heavy visitor traffic, sunscreen chemicals and warming water have damaged reefs at some of the most-photographed sites. Hanauma Bay (Oʻahu) limits daily visitor numbers specifically to help the reef recover, and it shows: it remains one of the most reliable spots in the state for both coral health and fish diversity.
Reef-safe sunscreen is not optional
Hawaiʻi state law restricts the sale of sunscreens containing oxybenzone and octinoxate, both linked to coral damage. Use a mineral (zinc oxide or titanium dioxide) sunscreen, and consider a rash guard for extended time in the water — it cuts down how much sunscreen you need at all.
Where the turtles actually are
Hawaiian green sea turtles (honu) are protected by federal law — keep a minimum distance (generally at least 10 feet) and never touch or chase one, however tempting the photo. Reliable, regular sightings happen at Punaluʻu Beach (Big Island, on the sand itself), Laniakea Beach (Oʻahu's North Shore, also on the sand), and while snorkeling at Kealakekua Bay (Big Island) and Honolua Bay (Maui).
Snorkeling vs. diving — which one
Most of Hawaiʻi's most famous marine life — sea turtles, reef fish, often even manta rays at the right spot and season — is visible well within snorkeling depth, no certification required. Diving opens up more: lava tubes, deeper wall dives, and Oʻahu's genuinely excellent wreck diving (including several intentionally sunk ships turned artificial reefs). If you're not certified, several operators run beginner-friendly introductory dives.
The Big Island's manta ray night dive/snorkel
Off the Kona coast, boats use lights to attract plankton, which in turn draws manta rays to feed at close range after dark — a genuinely unusual, memorable experience available to both certified divers and snorkelers, run by numerous licensed operators.