Common Mergansers are numerous in summer along rivers in northern North America, and many a canoe trip will turn them up without much trouble. Look for them sitting on rocks in midstream, disappearing around the next bend, or flying along the river, when their white wing patches and heavy bodies make them easy to identify. In winter, seek Common Mergansers on large rivers and lakes; look for them in large flocks mixed with other ducks such as Common Goldeneye and Bufflehead. Look for the sharp dark-and-white contrast of the snazzy males and the crisply defined, rusty heads of females.
(Cornell Labs, All about birds)