It really varies, we went one year and it was freezing (I'm told that's very rare) and another we were snorkeling etc and it was perfect. Average temp is suppose to be 80-83 with evenings being cooler in the 60's which for this Texas girl is a bit nipply.
We are going the last week of January this yearand figure if it's cold we'll just more time up in the hot tub.(they will open it up early if people are cold).
I will say there's a lot folks from up north when we go in Jan and Feb and of course they think I'm a whimp. Lol