Greece is a country with an archipelago of approximately 3,000 islands, 15.000 km coastlines and a very mild climate with sunshine throughout most of the year. The temperature of most Greek seas is about 14-16°C during the coldest winter months (January through March), with 14°C being suggested by the Open Water Swimming societies as the minimum water temperature at which wetsuits are optional.

Mass tourism in popular places comes to an abrupt stop at the end of October with the end of charter flights but the chances of good weather are still high up until the beginning of January, when it is still possible to swim in quite "normal" water temperatures (16-19°C). The Hellenic National Meteorological Service website provides information about the sea temperatures per region.

In the southern island of Crete the sea temperature up until late December is not lower than in June (18-19°C) while  the rainy days are fewer that in the rest of the country. 
 
Apart from the beneficial properties of bathing, pretty much known to all, open water swimming in winter improves the function of the immune system. The cool water stimulates the body and the quiet Greek winter beaches give a chance for a nice relaxing break. Greece, especially its southern part, is an ideal place to enjoy swimming even during the winter months of the year.
 
See also: Explore Cretan Beaches WebsiteWeather, Temperatures & Climate in Crete Archive; Video: Sea surface temperature - Crete