What property of water allows fish in a lake to survive winters freezing temperatures?

I think you are probably talking about the fact that frozen water (ice) is less dense than liquid water, so freezing temperatures will enable the ice to form on top of the waterbody, but still allowing liquid water underneath so fish can survive. Also, ice is usually translucent, so it can still penetrate the ice, allowing algae to continue to photosynthesize, providing oxygen for the fish below.