The poem puts us inside the moment a heart attack hits. It begins with a "great knocking," as if something is pounding on a door from within, and then the speaker is suddenly "down," stiff and helpless as wood. The body has stopped feeling like a living thing and become an object.
The middle lines turn to the sea. Hopes must be "settled on shore," as though they need solid ground, but the water is "almost at low water," so the tide is nearly out and fear feels certain. The image suggests life draining away, though that is an inference from the imagery rather than anything the poem states outright. The speaker can only "urge the surgeon on," handing control to someone else.
The last line is the saddest. The heart is "heavy for not hearing my heart," a strange, doubled phrase that hints at the pounding having gone quiet. The mood is tense, frightened, and oddly hushed, with helplessness running through every line.