Anxiety is the subject here, and the speaker feels it as something physical and predatory. Worry is a shark living in the stomach: it thrashes in tight circles, its fin cuts through the moonlight, and its teeth are "iced knives." The image of a "bathtub belly" suggests a small, enclosed space where the creature can't escape or be escaped from. The poem seems to take place in the sleepless dark, where the nights repeat and the mind spins.
As the poem goes on, the worry widens. It moves from the speaker's own body to the house, which rots, and then to the children, who will grow old and die. The speaker's gut seems to insist on feeling every possible loss in advance, demanding its "share of the pain." The final image, a stone crossing a river at rising tide, feels precarious and heavy, as if the speaker is barely keeping footing. The mood is tense, claustrophobic and exhausted, with a quiet dread that never fully lifts.