I've seen it as warm as the lower 80's during the day, and then I've seen it in the upper 30's/low 40's during the day to below freezing at night. One year we went to TL after Christmas. It was in the high 70's with a light cool breeze, a little bit too cool for me to go swimming, but my kids did. We usually go the week after Christmas but have gone earlier in the month. It can be a little warmer then than at Christmas.
When we go in December, we pack everything from shorts & T shirts to jeans, sweatshirts, jackets & gloves. It is best if you wear layers, as it is cool in the morning and tends to warm up during the day, but as evening comes, it can get chilly. It's hard to predict what the weather will be like, so I would rather be prepared for all possibilities.