The speaker is a submariner, and the poem moves through his dreams while he is shut inside a vessel under the sea. The first dream is of the sea itself, and the cod he imagines is "raw and horrid," a blunt, almost ugly image of nature. Then the setting shifts to a "green chamber," which could be the submarine's interior or the sea seen from within, with ducks and teals flying through it. Birds belong to open air, so their appearance in a sealed underwater space feels strange and dreamlike, a mix of longing for the surface and confusion about where he actually is.
The last two lines turn personal. He "could not reach" his wife, and he describes her as having a "thin astronomy," as if she were a faint pattern of stars far away. The mood is lonely and hushed. The speaker seems cut off, and the people and places he loves have become distant, dim points he can only glimpse in sleep.