The Boston Museum of Fine Arts has the perfect description:
“The Fog Warning is a painting with a narrative, though its tale is disturbing rather than charming. As indicated by the halibut in his dory, the fisherman in this picture has been successful. But the hardest task of the day, the return to the main ship, is still ahead of him. He turns to look at the horizon, measuring the distance to the mother ship, and to safety. The seas are choppy and the dory rocks high on the waves, making it clear that the journey home will require considerable physical effort. But more threatening is the approaching fog bank, whose streamers echo, even mock, the fisherman’s profile. Contemporary descriptions of the fishing industry in New England make clear that the protagonist’s plight—the danger of losing sight of his vessel—was an all-too-familiar event.(..)”
An account related in the 1876 history The Fisheries of Gloucester tells of the insidious horrors to which fishermen were prey and could well have served as a description of The Fog Warning:
“His frail boat rides like a shell upon the surface of the sea … a moment of carelessness or inattention, or a slight miscalculation, may cost him his life. And a greater foe than carelessness lies in wait for its prey. The stealthy fog enwraps him in its folds, blinds his vision, cuts off all marks to guide his course, and leaves him afloat in a measureless void.”