The wind turns to northern, instead of the usual western, and drives the
humidity away, while it cools the island. In Santorini, the sea
temperature is about 23-24 degrees these days, but, of course, the sea tends to
be warmer in beaches with shallow waters (like Monolithos) or later in the day.
The temperature seems rather cool for the Greeks, though for visitors from
northern or southern countries it is rather warm!
You can opt for beaches like Perissa, Perivolos or Vlychada, with no
waves today, unless you are an extreme sports fan!