previous poster is right, the sea is too cold, so make sure you have a place with a pool that is heated.
It's high season at Christmas, but if you can find a beach place that is right on the sand, and you can walk to restaurants and shops and things, it's wonderful. Its the best time of the year, because it is not too hot to sit outside and enjoy the sunshine.
If you are coming to pinellas, I will recommend nice places to sit outside and enjoy the water.