We have been to the beach in Florida in the winter. Somethimes, late November, once or twice in January. A lot depends on what you are used to, and what the weather is like at the time. It could be in the 70's then, or in the 50's then. You really never know. The old "crapshoot".
It also depends on what you are used to. Being from PA, we love going to the beach or pool when it is 75 outside. The native Floridians look at us like we are nuts! I would suggest that you make the reservations and count on the pool being heated. If you are going just for the ocean, you may be disappointed. If you are going for the resort, take your swimsuit and hope for the best.