The weather is awful this year!
If everything else was OK...
You were just unlucky, normally this time of year is IMO the best for vacation in the Islands.
In mid summer the heat and humidity and in August the winds are sometimes unbearable...
especially in the Aegean Islands though, not the Ionian you've visited.
If you went in Cephalonia by boat you must have visited my hometown Patras, (the port).
My house is just opposite Gate 1 of the old harbour, 500m from the dock of the Cephalonia ferry.
My grandfathers village is of the mountainous ones: Komitata.
I spent the whole season from May to October '07 in Cephalonia, restoring the wall paintings of a chapel near Agia Efemia.
I went for swimming to Myrtos every day!