Turtles breath through their butts in the winter.
No, really, I'm not kidding.
Some reptiles bury themselves in the mud and go into a torpor - where they are dormant - others like turtles are active in winter. Turtles are not as active in the winter because the temperature is lower, but they do feed and swim around.
Reptiles breath air just like we mammals do. But in the winter with ice covering the surface of most fresh water, s0 they cannot come to the surface. Turtles have sacs in their cloacas (the science word for reptile butt) with lots of blood vessels so they can take the oxygen directly into their blood from the water.
If you don't believe me, check this more detailed explanation.