The Goldeneye is a diving duck, with as its name suggests, a golden eye. Male and female are very different. The female, in the second photo has a grey body with a chocolate coloured head. She also has a distinctive dark bill with orange tip. One of the females has an all-black bill, she is a juvenile. Males are black with striking white patches on their side plus a white cheek spot. The black often catches the light giving him a green sheen. Goldeneyes spend quite a long time diving underwater, so can be a bit elusive. They also prefer deeper areas when in ponds and lakes. I rarely see them on the sea, which is where this flock was.