Brooke gives fish a theology, and the joke is how closely it resembles ours. Full of flies and drifting through a June afternoon, they ask the old question, "is there anything Beyond?" Their reasoning is wonderfully feeble: "This life cannot be All, they swear, / For how unpleasant, if it were!" Belief here rests on wanting, not evidence. The central idea is that our pictures of heaven tend to be our own world with its troubles removed, so the fish's paradise is simply "wetter water, slimier slime."
Much of the humour comes from vocabulary. The fish speak in solemn capitals (Stream, Pond, Beyond, All, Purpose, Faith) as though they were drafting a creed. The abstractions keep collapsing into puns. "A Purpose in Liquidity" turns a philosophical phrase into a statement about water, and "the future is not Wholly Dry" plays on "wholly" and "holy". Even the rhymes join in. In "Good / Shall come of Water and of Mud," the near-rhyme of "Good" with "Mud" quietly suggests that the fish's goodness is made of the same stuff as their mud. The word "somehow" in that sentence gives away how little argument lies behind the confidence.
The form supports this. The poem runs in tight eight-syllable couplets with a bouncy, nursery-rhyme beat, and most couplets are self-contained, each one a neat article of faith. That tidiness sounds like smugness, and it is interrupted only once. "Mud unto mud!—Death eddies near—" borrows the cadence of the burial service ("ashes to ashes") and turns it into pond language, and "eddies" makes death a movement in the water. The dashes and exclamation marks show a flicker of panic, and the fish answer it by repeating themselves: "Not here the appointed End, not here!" The insistence suggests they are afraid.
What follows is their God, and Brooke lets the King James register run beside absurd biology. "There swimmeth One / Who swam ere rivers were begun" sounds like scripture, but this deity is "of fishy form and mind," and the list of attributes, "Squamous, omnipotent, and kind," drops the word "scaly" in among the divine qualities. The line "under that Almighty Fin, / The littlest fish may enter in" is funny and also slightly touching, since the longing for a protector is real even when its object is ridiculous.
The fish's heaven is also a menu. It offers "Fat caterpillars," "Paradisal grubs," and "Unfading moths, immortal flies." The line "the worm that never dies" is a scriptural phrase (in the Bible it describes hell), and here it becomes a promise of dinner. The most revealing detail is the hook. "Oh! never fly conceals a hook, / Fish say, in the Eternal Brook." Look back to the opening, where the fish are "fly-replete," and the danger is already there, a hook inside the thing they love to eat. Their heaven is organised around the one threat their theology cannot explain, and the reader can infer someone on the bank holding the line.
The last couplet delivers the punchline: "And in that heaven of all their wish, / There shall be no more land, say fish." It inverts a line from Revelation, which promises "no more sea." For the fish, the worst thing is not death in the abstract but the dry world above the water, so heaven means abolishing it. The deadpan "say fish" leaves the reader to notice that the fish's whole idea of the divine has been about themselves.
This is why the poem matters. It is very funny, and it has teeth. Brooke is best known for solemn, patriotic war sonnets such as "The Soldier," so this poem shows a more sceptical, mischievous side of him. It is not simply cruel, though. By the end the fish seem less foolish than familiar, and the poem asks whether any of us imagines heaven except by taking away whatever we fear most.