Battery's freezing?

I must have some very uninformed batteries. I have a dual (starting) and deep cycle batteries for my boat, which both sat out the last couple of winters with our coldest temps being -15F. Come spring they both still had a charge and never froze. No, they were not hooked up to a trickle charger over the winter nor did I ever bother to check them.

The TT battery was kept charged and no problem with it either. It also set outside over the winter and I did check the power panel whenever I wandered into the trailer.