The Ionian islands enjoy the stable, predictable marinetime climate with fairly cool summers (typically 28 C- 34C by day, 22C - 26C by night) compared to other parts of Greece. However, exposure to the sun is deceptivly high, so protect your self!
Generally speaking rain is very unlikely, and apart from a regular afternoon breeze there are rarely any winds.
The sea temparuture is pleasant, though a few degrees cooler then the Aegean.