Feb is generally the coldest month down here in FL. Dec/Jan can be very variable.
I've been on the beach around Canaveral/Cocoa on Christmas day before in 90 deg, and got sunburnt, yet another year it's highs in the low 60s.
The gulf is always warmer than the atlantic, so SW Florida is a good bet if you can't get a spot in the keys.
We love Sanibel Island (near Fort Myers), and Siesta Key (near Sarasota), but anywhere from Clearwater down to Naples can be great if you're lucky with the weather.
2015 Rockwood Signature UltraLite 8281WS
2016 Ford F-250 Lariat CrewCab, 4x4, 6.2L V8