Beach
Heikendorfer Strand
Heikendorf, Germany · sea 18.6 °C at its warmest, in August

Sea temperature month by month
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020, sampled 28 km offshore, at the nearest open water cell of a 0.25° grid.
About
Beach at Kiel fjord
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 Heikendorfer Strand?
The sea off Heikendorfer Strand averages 18.6 °C at its warmest, in August, and 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 Heikendorfer Strand?
The sea here never averages 20 °C in any month, so swimming means cold water all year. Some people do it anyway.
Where is Heikendorfer Strand?
Heikendorfer Strand is on the coast of Heikendorf, Germany, at 54.3798, 10.1972. The map on this site flies straight to it.