December in Sea world????? They decorate the park with Christmas flowers and colors. They have a big tree of lights. They have used Norman Rockwell as the theme for a couple years. I don't remember them doing any special christmas shows etc. Now Busch Gardens on the otherhand has a great Christmas show where they have some great singers that do Christmas songs with a lot of interaction with the crowd.
It can be warm or it can be cold enough to need winter coats.