The oldest surviving dream manuals were written for a seafaring world, and they read the ocean accordingly. Artemidorus of Daldis, whose Oneirocritica was compiled in the second century, treats the sea largely through what people did on it: voyages, cargoes, harbours and storms, read one way for a merchant whose living crossed the water and another for a farmer who never would. His method, interpreting the same image differently by the dreamer's station and circumstances, is visible nowhere better than here.
The modern depth reading descends from a different source. C. G. Jung wrote repeatedly that water is the commonest symbol of the unconscious in dreams, and the sea, in the Jungian tradition, became the unconscious at its most collective and impersonal: the element every individual river runs into. Freud's tradition attends instead to the feelings the water carries, and to the sea's old associations with the maternal, readings his school derived from its theory rather than from any dream book.
Between the two stand the folk dream dictionaries, which read calm seas toward good fortune and rough ones toward trouble with cheerful confidence. All three registers survive in circulation, and they do not agree; the atlas records them as the traditions' readings, not as settled fact.