It wasn't like that for all of that December, though. There was a part of it where it was significantly colder. (and people who ran the Marathon in January of 2010, just a month after that December, said they started with it snowing on them)
The cold in Dec 2009 made me foolish for our first trip in Dec 2010. Thought that cold wouldn't happen again. Oh ha ha ha how wrong I was! It started off decently enough, but there was one morning of 29 when I thought I was going to freeze my head off. (that was the morning at Universal when my son had us ride Flight of the Hippogriff, early early early, 14 times in a row...after the 14th time I went right into the giftshop and bought $100 worth of scarves) The last day, as we were leaving for the airport, it warmed back up.
During the trip I mentioned above, we swam. Swam one night at CBR, swam over at Universal, swam at AKL when we had moved over there. The Universal swim was when it was the absolute coldest.
While you are in the water, the parts under the water are toasty. But if you get your hair wet, your head will freeze if it's supercold outside. And getting out of the water is absolutely rotten.
Remember, we swam. But it's not like it was fabulous. We did it to say we had done it, and to be in nice warm water after freezing our rears off all day. (but at Universal hubby sat on a chair and read his Kindle while we acted crazy LOL...had the towels all over him to stay warm, and he's a guy who wears shorts until it's below freezing)
BUT BUT BUT! That weather isn't normal. Go prepared with longjohns and warm socks (the thin, warm layers underneath would have been so much better than layering things on top like we tried), bring a variety of clothing, and hope for typical weather! Be prepared, though. Especially if anyone is an outlying sort of size where buying warm clothes would be difficult, as it would have been for us.