Hardheads are ducks and they're a smallish, (45cm) sleek and pretty bird with a blue tip at the end of their beak. (And although they were originally known as "White-eyed ducks" only the male has a white eye.)
They don't have a particularly hard head, but in researching them I found a couple of macabre references as to why they're named Hardhead. Firstly, because apparently in times past taxidermists found the head particularly hard to stuff and secondly, because hunters found them hard to kill!
Hardheads are diving ducks and can dive deeply and stay underwater for at least a minute at a time. I watched this pair for ages as they circled, diving repeatedly, or just upending themselves slightly and feeding on the water weed.