Janaury is not crowed, but the weather can be cool. Personally, we like January, but some folks may really want the warmer swimming weather. I have been there in January where you could swim, and when you couldn't.
The first week in february probably isn't crowded either, and it may be a little warmer overall than January. I think once you get into the 2nd and 3rd weeks in February, you're going to start hitting winter school break crowds.