Leigh,
What kind of sailing instruction are you looking for? If you have already been sailing three years, you already know how to sail.
We have a five-day sailing seminar for beach cats April 23-28 in Key Largo. Guaranteed to make you a better sailor.
As far as warmth in South Florida (which I consider to be south of Lake Okeechobee), the months most likely to have some REALLY cold conditions -- when a cold front comes through -- are January and February (really cold means mid to high 40's at night and mid to high 60's daytime).
But if your vacation is at least one week long, a cold front normally doesn't last longer than three days, and then the temp will go back up close to 80 degrees. So in a week you are usually guaranteed at least half a week of beautiful sailing weather.
At least that is how it works here in the Keys.