Beach
Plage des Grands Sables
Brittany, France · sea 18.5 °C at its warmest, in August

Sea temperature month by month
10.5Jan
9.8Feb
10Mar
11.6Apr
13.9May
16.3Jun
17.8Jul
18.5Aug
17.5Sep
16Oct
14Nov
12Dec
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020.
Wikipedia has no article about this beach yet. It is on the atlas from its Wikidata record and its photograph.
Questions
How warm is the sea at Plage des Grands Sables?
The sea off Plage des Grands Sables averages 18.5 °C at its warmest, in August, and 9.8 °C at its coolest. These are monthly means from NOAA's OISST v2.1 satellite and buoy record for 1991 to 2020, sampled at the beach's own coordinates.
When can you swim at Plage des Grands Sables?
The sea here never averages 20 °C in any month, so swimming means cold water all year. Some people do it anyway.
Where is Plage des Grands Sables?
Plage des Grands Sables is on the coast of Brittany, France, at 47.3158, -3.1000. The map on this site flies straight to it.