I watched and puzzled over this lone diving duck for many minutes. My field notes:
lots of white in front when it faced us
thick white stripe from the eye back, almost meeting in back of the neck
black nape and crest
grayish white on water line
light-colored bill that looked either unusually large or misshapen (flattish or wide)
Some possibilities I considered and discarded: Bufflehead with its crest unusually pinched, Horned Grebe that for some reason had an entirely white breast, Ruddy Duck/Hooded Merganser hybrid (is that even possible?)
Any other thoughts?