Terrible photo, but shows the very distinct border of dark and white on the neck of the Pacific to the rear of a Common. This was a young bird, with a slightly paler back than an adult. Not visible here, but the fine scalloping on the back was visible.

These birds give me the most trouble trying to get photos of anything I've tried, as a rule. This bird was probably 300-400 yards away, giving great looks, but this is the best I could do.