Common scoter are not unusual in Dorset in winter and there are always reports of them but in my experience they are often particularly difficult to see. This is because they are a sea duck coming only occasionally to estuaries or large lakes and lagoons near the coast. Most often they are some distance off shore and difficult to see and they continually go out of sight as they bob up and down on the waves or dive in search of food; they are usually just far off black specs on the water! However, all is not lost as they tend to gather in rafts and so there are several to see but they are also quite mobile and often fly together and that is when they are at their most obvious.
Common scoter are winter visitors seeking relief from the harsh conditions in their breeding grounds in northern Europe and in some northern areas of Britain. That said there are sparse sightings in summer too with non-breeding birds moving off shore here. There does not appear to be a pattern in the weekly reports chart but January and April seem to be favourable months for sightings but, in general, there are never a lot of records in a particular period but more consistent reporting over a long period.
There have been reports from Longham Lakes of common scoter but one would normally expect to see them off shore and Portland Bill tends to report them most frequently. Sea watching sessions elsewhere often produce reports too and occasionally they can be seen closer to shore in Portland, Poole and Christchurch harbours and at points along the Flett.
Once you know what you are looking for, where to look and when to look, adding common scoter to your Dorset list should not be too difficult!