The speaker confesses to a hidden hatred and compares it to a private sea, something vast that only they can see. The title, "Mare Frigoris," is Latin for "Sea of Cold," and it is also the name of a dark plain on the moon's surface, so the poem sets this inner sea in a cold, barren, faraway place. That fits the imagery that follows: ships "put out to rust" and a landscape that is nothing but desert. The hatred seems to have no way out, like vessels left to decay with nowhere to sail.
The poem suggests that this feeling is costly. The speaker imagines a voyage toward "white hair and ill health," as if nursing the grudge will age and wear them down. Yet the mind keeps going back to it, "full of how to get rid / of my enemy." The mood is bleak, tense, and a little claustrophobic. The short, broken lines make the hatred feel like something obsessive, circling in the speaker's head and never quite resolved.