Beach
Plaka
Greece · sea 25.2 °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.
About
tourist beach
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 Plaka?
The sea off Plaka averages 25.2 °C at its warmest, in August, and 15.6 °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 Plaka?
The sea reaches 20 °C or more from June to November. Twenty degrees is where most swimmers stop calling water cold; below it, a wetsuit helps.
Where is Plaka?
Plaka is on the coast of Greece, at 35.2969, 25.7273. The map on this site flies straight to it.