Bird Identification: Experts, your opinion on these swimmers?, 1 by MartyJo

MartyJo wrote:
We observed two separate flock of these as we came back across Iowa on small ponds. The light was bad for ids or photos, but some of you are so good. We thought not ducks; couldn't see pied bill for pied bill grebes. Couldn't see any white on front for loons, and we thought too small. Maybe cormorants? They're migrants in Iowa mainly in April and October, according to my book. Can you set us straight?

Thanks,
Marty