The coldest that French Mediterranean waters typically descend to is 12/13 degrees centigrade in the middle of winter. This means you can get away with a semi-dry in all but the foulest of “out of water” weather conditions.
The current trend is towards slightly milder temperatures particularly towards the end of the season. For example as late as mid November 2006 water temperatures were holding at 20 degrees. The following table gives a reasonably accurate comparison between UK and French Med sea temperatures year round (based on the Channel):
