When watching birds at sea one rarely gets a good view. The bird may be some way off, the light may be poor, the continual motion of the waves keeping the subject moving out of the field of vision of your binoculars and then, to cap it all, it dives beneath the surface! Despite that it is still often possible to make a positive identification through little signs.
Plumage colouration is only a part of the overall picture as I think this photograph shows. Obviously it is a 'duck' and it is on the sea so that narrows down the choice straight away and then, in profile, two key features are visible. The bird has a longish pointed bill, far from duck-like, and it looks like it needs a good feather brush to sort its head dress out! So with a distant view and no colours to go on this can still be named, a red-breasted merganser (Mergus serrator).
This is a form of duck, it is one of a family known as sawbills as they have a serrated bill to help them secure fish beneath the waves. They are nearly always seen at sea although good numbers can be found inside the confines of Poole Harbour and they come in good numbers from the cold north every winter.