Contrary to the saying that what acts like a duck must be a duck, here is one bird that swims like a duck, but is no duck! It's the American Coot, a somewhat strange looking creature with quite unusual legs and feet.
About the Coot, the site "All About Birds" says this: "Although it swims like a duck, the American Coot does not have webbed feet like a duck. Instead, each one of the coot’s long toes has broad lobes of skin that help it kick through the water. The broad lobes fold back each time the bird lifts its foot, so it doesn’t impede walking on dry land, though it supports the bird’s weight on mucky ground." See: http://www.allaboutbirds.org/guide/American_Coot/overview