Nantasket Beach Water Quality

Yes, you can swim at Nantasket Beach, and its water is among the cleanest of the Boston-area beaches. In the 2025 season every water sample DCR took there was within safe bacteria levels, a 100% rating in Save the Harbor's annual report card, up from 92% in 2024. Before you go in, check the state's beach water quality dashboard for that day's status.

How to Check If Nantasket Is Open for Swimming

The Massachusetts Department of Public Health (DPH) runs the Interactive Beach Water Quality Dashboard. During beach season it updates hourly and shows each beach as open or posted. Choose Hull as the town, then the Nantasket listing, to see its status and recent test results. DCR also posts results on its Park Alerts page.

A posted beach is closed to swimming, not closed to visitors. You can still walk the shore, sit on the sand and play on the beach. Just stay out of the water until it reopens.

How the Water Is Tested

DCR starts testing its beaches each May, just before the season opens; in 2026 it began the week of May 18. At ocean beaches like Nantasket the test looks for Enterococci, bacteria that signal the water may be contaminated with sewage or animal waste. Samples are taken in about three feet of water, a foot below the surface.

The state's limit for an ocean beach is 104 colony forming units per 100 milliliters in a single sample, and an average of 35 across the last five samples. Most beaches are posted after two high results in a row; beaches with a history of problems are posted after one. The details are in DPH's beach testing FAQ.

There's a catch. Lab results take about 24 hours, so a posting is always based on yesterday's water. A beach can reopen a day or two after a bad result, once a new sample comes back clean.

Nantasket's Water Quality Record

Save the Harbor/Save the Bay grades the 15 public beaches from Nahant to Nantasket every year using DCR's test data. The rating is the share of samples that came back within safe levels. Nantasket scored 92% in 2024 and a perfect 100% in 2025. That same year, for the first time, every beach in the report scored above 80%.

When to Skip a Swim

• After heavy rain: Save the Harbor advises waiting 24 hours after a heavy downpour (more than 0.3 inches an hour). Rain washes runoff into the water, and DPH says bacteria levels tend to rise after it.
• Near pipes and outlets: DPH says to avoid swimming next to drain pipes and to stay away from discolored, fast-flowing or strong-smelling water.
• With open cuts: DPH warns that cuts, scrapes and new tattoos or piercings raise the risk of Vibrio infection in coastal water.
• In rough surf: beaches can also close for rip currents or poor visibility. Double red flags on a DCR beach mean it's closed to swimming. Our Weather page covers storms and surf.

Lifeguards and Swim Safety

Lifeguards are on duty Memorial Day to early September (weekends only until June 18). Once full summer coverage starts, DCR guards its beaches from 10:15 a.m. to 5:45 p.m. seven days a week, depending on staffing. Swim only in the designated swimming area, and if you can't swim, stay in shallow water and wear a U.S. Coast Guard approved life jacket. Season dates are on our Hours page and the full rules on our Rules page.

Check the tide and the water temperature too. At high tide much of the beach is under water, and the ocean here averages only about 66°F in July.