Beach
Matala
Tympaki municipality, Greece · sea 25.4 °C at its warmest, in August

Sea temperature month by month
Monthly means in °C, NOAA OISST v2.1 climatology 1991–2020, sampled 17 km offshore, at the nearest open water cell of a 0.25° grid.
About
tourist beach on Crete with caves/tombs occupied by hippies in the 1960s
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 Matala?
The sea off Matala averages 25.4 °C at its warmest, in August, and 16 °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 Matala?
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 Matala?
Matala is on the coast of Tympaki municipality, Greece, at 34.9938, 24.7495. The map on this site flies straight to it.