Beach
Praia do Martinhal
Vila do Bispo, Portugal · sea 19.7 °C at its warmest, in September

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 Praia do Martinhal?
The sea off Praia do Martinhal averages 19.7 °C at its warmest, in September, and 15.2 °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 Praia do Martinhal?
The sea here never averages 20 °C in any month, so swimming means cold water all year. Some people do it anyway.
Where is Praia do Martinhal?
Praia do Martinhal is on the coast of Vila do Bispo, Portugal, at 37.0185, -8.9265. The map on this site flies straight to it.