I'm with Castlemaid on this one!
Living in Ottawa we get all kinds of cold weather. In early March of this year, Kyleigh was swimming in the thawed out parts of the river!
Just last week, it was only 2 or 3 degrees above zero, and she still went swimming - and I mean swimming, not just running through water!
I too carry towels in my car to dry her off, but I also get her running around when she comes out of the water to keep her blood flowing!!
She loves it and I've seen no ill effects from her at all.