Hey Rima,
We were at BWV from the 3rd thru the 10th this year. Weather is a total toss-up! (just what you wanted to hear!) It was chilly for the most part, a coupla days in the 70's, pouring rain one afternoon (while we were on the Jungle Trek at AK, nowhere to hide!) But, I heard the next week stayed in the 70's almost every day! I did go swimming one day at Luna Park and it was fine. It was a bit nippy and overcast but I pretty much had the whole pool (and hot tub) to myself!
As far as crowds go, you should be just fine. We noticed more people during the first few days (still 1st week of Jan.) and many less on the end of our trip. I'm sure this was a combo of new years and the Marathon. If you're there for 13 days you should be in heaven! Check to see if MLK weekend is during your stay. Take advantage of an E-ride night (but rest up, we bought tickets for our last night and were too pooped to go by that time, our friends went and had a great time though!)
My only regret about this last trip was that I wanted it to be perfect (we took some newbies) and ended up frustrated at times. Just relax and enjoy whatever comes your way!
Have a great time!!!!