Swans have a large number of feathers, including down feathers which protect them from even sub-freezing temperatures. The water temperature is usually warmer than the surrounding temperature (think about going into water on a cool day- once you are in you don't want to come out).

While in the water, the swans use the warmer temperature to heat their feet which have a large number of blood vessels. These capillaries promote temperature regulation for the swan's body. The marshy wetland areas that you mention provide a great habitat to protect the swans from wind. The swans will use the grasses to snuggle and provide some shelter, but the grasses will also keep the birds in a small area during heavy gusts so they do not have to fight against the wind to stay on the river.

The Regal Swan® Foundation, Inc.