In just a few lines, the speaker describes a home gone wrong and a strange, almost desperate act. The "foul water" in the house suggests something rotten or unhealthy at the center of the speaker's life, and the image of "a ship on a fen" makes it worse: a ship belongs on open water, but a fen is marshy and still, so the vessel is stuck, going nowhere. The speaker seems trapped and out of place.
Then comes the odd turn. The speaker "forces" a doctor to fetch their heart, and the doctor is made to do it "for his salary," as if it were simply part of the job. The tone is dry, even darkly funny, but the feeling underneath looks like numbness or exhaustion. Judging by the title, "Heartless," the speaker may want to be rid of feeling altogether, or may be asking to have a lost heart returned. Either way, the mood is stagnant, uneasy and quietly desperate.