The ringed-neck duck is one of the birds that reminds me that many species were originally named by hunters. Looking at them I would have been more likely to call them ring-billed ducks because of the unique markings on their beaks. So why ring-necked.
Its not something that you see often out in the wild while they are swimming. Often they have their heads sort of tucked into their bodies so you can not see their necks, like in the first photo. However once in a while they stretch out their necks and then you can see the lighting band on the neck, as in the photo above. The neck ring is much more obvious when you have a dead duck laying at your feet, or so I am told. So as hunters killed these ducks they named them ring-necked because to them it was an obvious field marking.