They know when to get out and dry off...they're body is capable of redirecting bloodflow to areas that are coldest, and they have an inch or more of insulation that works very well to protect them. Water does not actually penetrate so it never touches their skin.
As long as they have some dry shelter to go to at night or in extremes they can hang in the pond or snow for hours. Food is the most important followed by dry shelter.
I watched our pond ducks sitting by the water asleep when it was in the mid 20s a cpl weeks ago...and others casually paddling around in the pond.