Can I swim yet?

Modeled sea-surface temperature for the water just off the beach (Open-Meteo marine model, NOAA and ECMWF inputs) · The nearest physical instrument is the Cape Cod Bay buoy, NDBC 44090, which reads the deeper water offshore

The verdicts, for the record

Under 50 is no. Fifty to 55 is not swimming, it is a dare. Fifty-five to 60 is wetsuit people only. Sixty to 65 is brisk; you will make a noise, and the noise is part of it. Sixty-five to 68 is honest New England swimming. Sixty-eight to 72 is genuinely pleasant. Anything above 72 is bathwater by our standards, and you should go in immediately because it will not last.

What the number doesn't tell you

The beach has two kinds of water. The ocean side takes its temperature from Cape Cod Bay, deep and slow to warm; that is what the number above describes. The bay side is a different proposition: on a sunny afternoon the flood tide comes in over miles of sun-baked flats and arrives noticeably warmer than the ocean it came from. Locals with small children learned this generations ago. If the number above looks grim, an afternoon flood on the bay side is the loophole; the tide chart tells you when it runs.

The season runs later than the calendar suggests, both ways. June water is colder than June air ever admits, because the bay is still paying off winter. The warmth peaks in late July and August, and then, the good secret, it holds: September water is often warmer than June's, with the crowds gone. The year's best swims are frequently had by people back in school-year clothes on the walk down.

One respectful note: water in the fifties takes your breath whether or not you consider yourself hardy. Wade in rather than dive, and give yourself a minute. The bay has been here longer than you and is not impressed.