White sand
Plage du Grand-Rohu
Saint-Pierre-Quiberon, France · sea 18.3 °C at its warmest, in August

Sea temperature month by month
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020, sampled 13 km offshore, at the nearest open water cell of a 0.25° grid.
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 du Grand-Rohu?
The sea off Plage du Grand-Rohu averages 18.3 °C at its warmest, in August, and 9.6 °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 du Grand-Rohu?
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 du Grand-Rohu?
Plage du Grand-Rohu is on the coast of Saint-Pierre-Quiberon, France, at 47.5112, -3.1240. The map on this site flies straight to it.