See this post regarding gulls and loons. The answer will be the same for scoters, long-tailed ducks, and red-breasted mergansers. Immature sea ducks regularly summer in bays and estuaries along the coast, well outside their breeding range.
As I hinted in this post, the way range maps are formatted is much more suited to migratory songbirds and doesn’t really capture the more nuanced ranges of waterbirds. In many distribution descriptions for these species (e.g., Birds of the World), you’ll see the categories Breeding Range, Nonbreeding Range, and Summer Nonbreeding Range, the latter is seldom expressed on range maps.