Many sailors from The Tampabay area on South sail all year with a wetsuit and spraysuit or a drysuit reccomended from Dec-Feb. Gulf water temperatures average:
Spring 73.7F, Summer 84.7F, Fall 76.7F, Winter 63.7F.
Air temperatures:
Jan Feb - 60's
Mar Apr - 70-80's
Jun-Oct - 80-90's
Nov - 70-80's
Dec - 60-70's
During the winter months we do get some cold spells that get down into the twentys for the lows but they usually only last for a few days before going back up to the 60's.
I think March/April is the best time of the year for sailing. Good winds, little to no rain, and moderate temps.
Good Luck,