Goosander nest throughout upland Britain in Scotland, northern England and Wales. They can be found on rivers and lakes in these regions, usually near woodland and forests as they like to nest in holes in trees although do also nest on the ground. They are diving ducks of the sawbill family and feed on fish and they are now in trouble in some Scottish salmon rivers and the local fishermen want their numbers 'controlled'; that is to say they want them dead! It is estimated that there are over 2,000 breeding pairs in Britain and they are not great travellers, they are forced to move south when the going gets tough up north and that seems to be where most of the Dorset birds come from. There may be an influx from elsewhere in northern Europe if conditions there get severe.
It is unlikely that goosander will be seen in Dorset after the end of April until the autumn arrivals start in late July. It is interesting that there are a few reports of goosander in Dorset from week 27 to week 33 and then they are absent until week 40 at the end of October but it is from week 44 at the beginning of November that the bulk of the sightings occur. After this surge in numbers there are then are ongoing intermittent reports throughout the winter. They are never common and are always considered of interest and worthy of reporting.
Most reports come from open water in Poole Harbour and from the Fleet at Ferrybridge. Outside of these two sites reports are somewhat scattered and birds can turn on rivers and lakes almost anywhere in the county, especially from along the river Stour and also the river Frome.
Predicting a way of getting goosander on to your Dorset list is tricky. You could try one of the Birds of Poole Harbour winter 'Bird Boats' as they sometimes come up trumps.