We are planning on January 12-19
Just mentioned on another thread, my kids school often does a five day weekend on MLK weekend. What I have noticed the past few years, the crowds have crept up a bit over that weekend. So, I would say that Saturday and Sunday were busy, levels dropping Monday evening, and rather light Tuesday and Wednesday. Can't tell you first hand much beyond that, we are normally gone after lunch on Wednesday. I would imagine low crowds the rest of the week.
Having said that, weather can be a toss up. You can wind up with a rather warm day, but nights get rather chilly, Not swimming weather to me, but you do see people in the pools. the average high is 72, with a low of 52, but that is the average. I have been there when they have had to close the water parks due to cold temperatures.
Hours - keep in mind, this is an off peak time, and the parks close rather early. Sunset is before 6:00 PM, and the parks normally close between 7 and 9.
Rehab - This is the time when a lot of work is done, Christmas is taken down, and the world switches to "Normal Mode". Do not be surprised that rides may be down, construction barriers may be up, etc.
Having said all of that, it is a great time to go.