"Water" begins with a hypothetical situation: "If I were called in," the speaker says, indicating that the poem takes place in the world of the imagination. "Water" is a poem about (you guessed it!) water and religious ritual, but it's also about possibility and, perhaps, hope.
The speaker specifically imagines being summoned to "construct," or create, a religion. Right away, readers might wonder: who or what would call on this speaker to make up a new religion, and why? Who is the speaker, and what makes them qualified to invent a religion (no small task!)?
The poem never answers these questions, but in raising them it does imply that religion isn't exactly natural; instead, it's something constructed, something made up by people. Whether this makes religion less valid or not is something that the poem ultimately leaves open to interpretation.
The first three lines also establish a few key stylistic aspects of the poem. "Water" is written in free verse, meaning it doesn't follow any set pattern of meter or rhyme. The first two lines in this stanza are also enjambed, flowing smoothly down the page. Along with the use of free verse, these enjambments grant the poem a casual, conversational, and flexible feel. Readers might get the sense that the speaker is simply musing, delivering thoughts off the cuff.
At the same time, the use of tercets—three-line stanzas—lends the poem some structure, and might subtly nod to the deep importance of the number three in various religions and mythologies (think of the Holy Trinity, the Three Fates, etc.).
The poem's seemingly matter-of-fact tone makes the speaker's true feelings a little hard to read. Is the speaker being sincere about "construct[ing] a religion" and basing it on water? Or is the speaker is being tongue-in-cheek, disingenuous, and sarcastic? The whole scenario does seem a little odd, and maybe even silly. And Larkin is famous for masking irony and emotion under a plain, even reserved linguistic surface. That skill is definitely on display in "Water," which asks readers to decide just how earnest or sly—or some combination of the two—the speaker's words about religion really are.