It depends on when you'll be going in Jan too. We were there early in Jan and there were crowds the first week then tapered off after the marathon weekend. They picked up again when we were leaving due to the MLK holiday weekend.
I wouldn't say the parks were crowded as opposed to the busier seasons (like summer or holiday time) but they definitely were slightly more crowded at the times I mentioned.
Also, the first couple of days we were there it was cold, and we are from NY so we're used to it, but colder than we anticipated IMO. We wore our winter coats, hats and gloves because it was pretty windy at night. After that it warmed up and we were able to swim a couple of times, but not so hot that we felt we could walk around in shorts all day. We tried it once and froze and we like it cooler, so just come prepared and dress in layers so that you can take a layer off or add one on if necessary. HTH!
Gl with your planning!