The poem watches the Thames tide as it moves among the ships, and it turns that movement into a small, playful scene. The rising water becomes a captain going from ship to ship the way a man might go from room to room at a tavern, knocking and calling in. It's an easy, sociable picture: the river as a place of visiting and chatter.
Then the poem takes an odd turn. The room the captain calls at holds "infinite cake," a line that feels like a dream or a child's idea of plenty. The ending pulls back to the plain facts of water in the reeds, and then to "a voyage into some bed." That could suggest the tide settling into its channel, or a sleepy drift toward rest.
The speaker never states a feeling outright. Judging by the imagery, though, the mood is warm, whimsical and drowsy. It's less about describing the river exactly than about letting its motion slide into something like a happy half-dream.