It's a common loon in winter plumage. The overall coloring (black on the back, white underneath) and the bill (heavy and daggerlike) make it a winter loon, and the white indentation on the neck further identifies it as a common loon.
The bill length was throwing me for a minute (it's not long enough,) until I realized that his head is turned slightly away from the camera.
Mergansers have very narrow bills, and different body and neck patterns.