Hanauma Bay Water Quality

Hanauma Bay is in a league of its own on Oʻahu. Formed in the crater of an ancient volcanic cone, the sheltered bay has been a marine life conservation district since 1967 — one of the first in the United States. The combination of natural shelter, protected status, and strict visitor management means the water quality here is consistently among the best on the island.

DOH monitoring at Hanauma Bay has historically shown low bacteria counts. There are no outfall pipes or storm drains directly feeding the bay, and no boat traffic that would stir up sediment or introduce pollutants. After heavy rain, upland runoff can temporarily affect water quality, but the bay flushes well.

Hanauma Bay’s sheltered, naturally enclosed bay with no boat access and strong open-ocean flushing produces consistently low bacteria counts in DOH monitoring. The bay closes on Tuesdays and some Mondays specifically to allow the reef and water to recover from visitor pressure.

Water clarity is exceptional on calm days — 30 to 60 feet of visibility is common. The primary water quality risk is after heavy rain, when stormwater from the Niu and Hawaii Kai areas can carry runoff toward the bay. On days following significant rainfall, the bay’s managers may reduce visitor numbers or issue cautions.

• Watch the 9-minute educational video required at entry — it covers reef etiquette
• Use reef-safe mineral sunscreen only — rangers enforce this
• Never stand on the coral — one step can kill decades of growth
• Don’t feed fish — it’s illegal and disrupts natural feeding behavior
• Stay in the sandy channels when possible to avoid coral contact
• Snorkel gear rental is available at the beach; bring your own for a better fit
• The outer reef has stronger currents — stay in the inner bay if you’re a beginner
• Green sea turtles (honu) are common — stay at least 10 feet away, federal law