Month-by-month breakdown of bacteria risk, rain patterns, and ocean conditions — so you can plan your trip around the cleanest water
Dry season across most islands. Less rain means less runoff, which means cleaner ocean water at the vast majority of beaches. This is when Hawaii has the fewest brown water advisories and the lowest bacteria test failure rates statewide.
The transition between dry and wet seasons. Rainfall starts picking up, particularly on windward and north-facing coasts. Water quality begins declining as streams carry more runoff to the coast.
Peak wet season. This is when Hawaii experiences the most rainfall, the most brown water advisories, and the highest bacteria levels in ocean water. Every island is affected, though leeward coasts are significantly less impacted than windward and north-facing shores.
Still wet, especially on windward sides. Kona low season overlaps with spring — March 2026 saw major storms across multiple islands. Water quality is improving but remains risky after rain events. North shore swells are tapering off.
Hawaii’s water quality varies dramatically based on which side of the island you’re on. Leeward (west-facing) coasts are drier and cleaner year-round. Windward (east-facing) and north-facing coasts get more rain and have more runoff. Here’s how each coast type performs:
Regardless of the season, the single most important rule for Hawaii beach water quality is this: wait at least 72 hours after heavy rain before entering the ocean.
Rain is the trigger that flushes bacteria from cesspools, animal waste, and urban pollutants into the ocean. The water may look clear again within 24 hours as sediment settles, but bacteria can persist much longer — especially near stream mouths, canal outlets, and enclosed bays.
This rule applies year-round but is most relevant during the wet season (October through April) when rain is more frequent and heavier.
The brown color from storm runoff clears before the bacteria does. Always wait the full 72 hours even if the ocean looks normal again.
Hawaiian box jellyfish follow a predictable lunar cycle. They arrive on south-facing beaches (Waikiki, Ala Moana, Poipu) approximately 8 to 12 days after each full moon. This pattern occurs year-round, not just in one season.
The influx typically lasts 1–3 days. Lifeguards often post warning signs, but not always. Planning around the jellyfish calendar can help you avoid them entirely.
Dry summer months (May–September) are the best time for swimming on Oʻahu. During winter, avoid Waikiki and Ala Moana after heavy rain — the Ala Wai Canal flushes urban runoff and sewage overflow directly into the south shore. The Kailua and Lanikai area is affected by Kawainui Marsh drainage after rain. North shore beaches are flat and calm in summer but have massive, dangerous waves in winter.
Best year-round option: Ko Olina’s lagoons on the dry leeward coast — enclosed, man-made, and rarely affected by runoff.
South Maui (Wailea, Makena) has clean conditions year-round — it sits on the dry leeward side and has minimal stream exposure. This is one of the most consistently clean coastlines in all of Hawaii.
West Maui (Kaʻanapali, Lahaina) is more variable. Multiple streams discharge into the nearshore waters, and winter rain increases bacteria risk. The area was also impacted by the 2023 Lahaina fire, which changed runoff patterns.
Best year-round option: Wailea and Makena on the south shore.
The Big Island has the most dramatic climate split of any Hawaiian island. The Kohala Coast on the dry leeward side (Hapuna, Mauna Kea Beach) receives very little rainfall and has clean conditions year-round. It’s one of the most reliable coastlines in the state.
The Hilo side (east coast) is one of the wettest places in the United States. Beaches on this side have consistently higher bacteria risk, and the risk increases significantly during the wet season. Kahaluʻu Beach on the Kona coast has chronic contamination from cesspool pollution that is separate from rain events.
Best year-round option: Hapuna Beach and the Kohala Coast resort beaches.
Kauaʻi is the wettest Hawaiian island. Mt. Waiʻaleʻale near the center receives an average of 450+ inches of rain per year — one of the wettest spots on Earth. The north shore (Hanalei Bay, Tunnels, Kēʻe Beach) gets heavy rain, especially October through April. Hanalei Bay is impaired by four river and stream discharges plus 360+ cesspools.
The south shore (Poipu) is dramatically drier. It sits in the rain shadow and has much better water quality year-round. Kalapaki Beach near Līhuʻe has failed 100% of bacteria tests since 2016 due to Nawiliwili Stream contamination — avoid regardless of season.
Best year-round option: Poipu Beach on the dry south shore.
Free alerts when water quality changes at your beach — brown water advisories, bacteria warnings, and all-clear notices.
No spam. Just safety alerts for your trip.
100+ beaches and 25+ hotels across all 6 islands
⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent passion project — it is not affiliated with the Hawaii Department of Health or any government agency. Water quality ratings on this site are estimates based on publicly available testing data and geographic analysis. They are not real-time measurements and may not reflect current conditions. “No DOH Alerts” means no advisory is currently posted — it does not mean the water was tested and found safe. DOH only monitors a fraction of Hawaii’s beaches, and some areas have no regular testing at all.
Always verify current water quality conditions with the Hawaii Department of Health Clean Water Branch before entering the water. This site is for informational purposes only and should not be the sole basis for any swimming decisions.
This site is a work in progress and we want to make it better. If you notice something that isn’t working right, have a suggestion, or want to share local knowledge about a beach, please reach out.
When in doubt, don’t go out.