As the weather may be bad, I'd choose one where you can "do something" if it rains (rain in winter mat last long). Thus, I'd choose a semi-large one with a "big" city (e.g. Corfu, Rhodes, Lesbos) and also plan a rental car to see as much as you can.
PS My wife goes swimming in February, but she's an exception. Water temperature in Crete is ~16-17 oC.