adamfromcanada's Identifications

They were diving fully underwater (not dabbling). Very confused by the combo of the white near the end of the wing (unlike ruddy ducks), plus the large beak and flat forehead (unlike buffleheads), plus the white on the cheek rather than near the beak margin (unlike scaups).

Bufflehead seems closest in terms of where the white markings are, and it seems that sometimes they can have a very large-beaked and slanted forehead appearance (though these examples aren't as extreme as the ones in this observation): https://www.inaturalist.org/observations/20900558 and https://www.inaturalist.org/observations/217621299