Prolonged sunshine and high temperatures during the first half of October have kept the sea surface temperature high, especially in the Ionian Sea, the Dodecanese and the northern Aegean.
At the same time, the northern winds blowing (atypical meltemi) in the central parts of the Aegean keep the surface temperature of the sea very close to normal for the season.
The map below shows the sea surface temperature, as recorded on Tuesday 15/10/2019, as well as its deviation from the normal values for the season. The contrast between the Ionian and the Dodecanese is clearly evident in relation to the Central Aegean and the Cyclades.
PS I can confirm this. The sea is more pleasant in October than it was in early June.