Last year in october i spent 5 days near L'ile Rousse (close to Calvi/CLY) and i was able to swim into the sea every day: sunny weather around 25°
Last month (in september) i spent 10 days near near Propriano and unfortunately i only took a quick dip as the sea was quite cold. It was almost raining every day during the first week...
Next year i will try june...

Regarding a nice location for a villa rental, if you are ok with not being close to the sea (20 minutes from it) you should try Santa Lucia di Talla (Sainte Lucie de Tallano), very nice and typical village corse (some shots in the movie "L'enquête Corse")