The speaker begins by correcting a common image. Depression is often described as a wall, but walls block sound and sight, and the speaker says depression doesn't. You can still see out. A better comparison, they say, is being adrift at sea: nothing to hold on to, and the shore slowly slipping away.
The twist comes in the second half. The walls are something the speaker builds on purpose, as a way of pretending to be on land. If you can't see the shore getting farther off, the loss hurts less. So the wall isn't the illness itself but a defense against feeling it.
The mood is quiet, tired, and honest. The short, plain lines and lowercase sentences feel like someone thinking aloud rather than performing. The title, "comfort," lands with some sadness, because the comfort is partial and self-made, a way of not looking at the water. The poem doesn't offer a rescue, only an understanding of why someone might choose the wall.