Great Crested Grebe

I know it's not strictly speaking a duck, but it swims and fishes in the castle moat. So for me it qualifies, it was happily fishing in the moat today and having more luck than the fishermen !

The Great Crested Grebe is the largest member of the grebe family found in the Old World, with some larger species residing in the Americas. They measure 46–51 cm (18–20 in) long with a 59–73 cm (23–29 in) wingspan and weigh 0.9 to 1.5 kg (2.0 to 3.3 lb).[2][3] It is an excellent swimmer and diver, and pursues its fish prey underwater. The adults are unmistakable in summer with head and neck decorations. In winter, this is whiter than most grebes, with white above the eye, and a pink bill. It is the largest European grebe.

The young are distinctive because their heads are striped black and white. They lose these markings when they become adults.

The Great Crested Grebe breeds in vegetated areas of freshwater lakes. The subspecies P. c. cristatus is found across Europe and Asia. It is resident in the milder west of its range, but migrates from the colder regions. It winters on freshwater lakes and reservoirs or the coast. The African subspecies P. c. infuscatus and the Australasian subspecies P. c. australis are mainly sedentary.