In the water, the harbor seal is a graceful swimmer. They propel themselves by lateral undulations of the hind flippers, which expand when swimming. The front flippers are used for an occasional guiding stroke. The harbor seal's fur is stiff, with no undercoat in the adults. Harbor seals lack the extreme sexual dimorphism common to many species of pinnipeds.
Harbor seals inhabit temperate and subarctic waters of the North Atlantic and North Pacific oceans.
In the North Pacific Ocean, harbor seals can be found from Alaska south along the coast of North America to Cedros Island off the west coast of Baja California and along the Asiatic coast to China. In the North Atlantic they are present from Greenland south to North Carolina and from Iceland south to the shores of the Netherlands and occasionally France. Harbor seals are littoral in distribution and can often be seen on protected tidal rocks and reefs. Groups may range from a few individuals to as many as two or three hundred, but they do not exhibit the sort of social behavior characteristic of sea lions, fur seals, and elephant seals. Harbor seals often come into bays and estuaries and may be seen resting on sandbars at low tide.