We've been in January for 10 of the past 11 years, we either go the 3rd or 4th week of the month.
One year, 2001, was very cold every single day. Yes, we live in CA and aren't used to it, but we definitely wore hats, coats and gloves every day. After dark, it was unbearable.
This year, 2009, was very warm. We swam every single day of the week.
2008, it was cool enough to be unable to swim at all but it wasn't uncomfortably cold except for one day.
All other years, we have generally had one day out of the week that was warm enough to swim and/or go to whichever water park was open (in Jan only one of the two waterparks is usually open). Hardly ever has there been more than one nice day within a 7 day vacation.