Ducks

They've been at the river for the past few days. I first saw them resting in the shade of a vehicle in the parking lot at the down river end of the walk. They are obviously used to human activity - they don't fly away like the mallards. I've no idea what they are (I've learned they are hybrid. Ducks are like that). They are larger than the mallards. The feathers on the back of one seem quite wet and untidy. Could it be unwell?

EDIT 6 July '19: I only ever see the duck with the white neck now the one I thought was sickly. Still very friendly. I wonder if it misses its companion?