Spent Christmas in 2003. arrived on 20 thDecember, got up the next day to bright sunshine ,Blue skys. Put on shorts and a T shirt went to sea world. It was sunny all day but very cold out of the sun, Got wet by Shamu, got even colder. Stayed till 9pm and was frozen daft. Americans from the south came up to me as we left the park and they were wearing fur coats, hats etc. They asked if I was from the North and I explained that I was from the North of England!
Thankfully the weather improved the next day and got warmer as the holiday went on, temps in 80's.
Going again on 17th December this year. The Answer really is be prepared for everything, If it gets cold buy some warmer clothing when you get there, The sales are unbelievable from boxing day onwards!