Portland Harbour supports a small population of Great Northern Divers every winter. Occasionally these are joined by a Black-throated Diver or two. Both species are very similar and can be a real headache to separate at a distance or in bad weather. This winter there have been more sightings of Black-throats than usual, including this one that has often been seen close in off Hamm Beach near the Sailing Acadamy.
A Great Northern Diver has been added at the end of the video for comparison. Here's the original sequence.