For me the 'benchmark' for identifying ducks is the mallard (Anas platyrhynchos). It has been so successful in our modern world you will find it all across Europe and in other parts of the world too. Mallard can be found anywhere there is water (salt and fresh), anywhere in Dorset (both inland and coastal) and any time of year (winter and summer) and so, if you see a duck it is, statistically, most likely to be a mallard.
I chose this photo because it shows very clearly the blue feathers in the wing; this is important because whilst both male and female are different in plumage they both have the blue in the wing. In late summer the male moults and loses its gorgeous metallic green/blue head but usually the blue in the wing is still visible. To add to the confusion mallard inter breed with some forms of domestic duck and all sorts of hybrids may be encountered but, even so, quite often the blue in the wing remains as clear indicator that you are looking at a form of mallard.
There are other unique features too one of which is their classic duck 'quack, quack, quack' call. The whitish flanks under the wing are quite clear, especially when in flight and so too is the white in the tail. Get to know your mallard well, then you will know when you are looking at a duck that is not!
I would have thought that the mallard would be one of the first birds on your Dorset list!