Here is a Tufted duck, on the ponds of Hampstead Heath. You can see why it's called a Tufted duck! The female (it's a male above) is much more drab, being brown all over rather than black and white. It is a diving duck, so makes regular trips below the surface. You can see its legs and feet under the water in the picture above.

They are fairly common but as with most such things it seems that you either have them on your local watery spots or you don't, so you'll be familiar with them or not. Personally I don't tend to see that many of them.