Figure 1. Photo by Kerry Ross. Notice the dark upperparts, clean contrast between the neck and throat and large bill. Even with the water in front of the bird obscuring most of it's body you can see the distinct white flank of the bird, shown throughout the observation.
Figure 2. Bird moving away showing distinctly white flank patches. Notice the dark back and dark grey color of the head and neck.
Figure 3. Arctic Loon diving. Notice how the white seems to curve up to the dorsal surface of the body. Good quiz photo!
Figure 4. Another Arctic Loon quiz photo. See the white. This is basically what the view of the bird was most of the time.