Beach
Plage de Tronoën
Saint-Jean-Trolimon, France · sea 17.6 °C at its warmest, in August

Sea temperature month by month
11Jan
10.3Feb
10.2Mar
11.5Apr
13.5May
15.7Jun
17Jul
17.6Aug
16.8Sep
15.7Oct
14Nov
12.3Dec
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 de Tronoën?
The sea off Plage de Tronoën averages 17.6 °C at its warmest, in August, and 10.2 °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 de Tronoën?
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 de Tronoën?
Plage de Tronoën is on the coast of Saint-Jean-Trolimon, France, at 47.8517, -4.3477. The map on this site flies straight to it.