There's a handful of alliteration in "Water," which lends this short poem some lyricism and intensity. In the first two lines, for example, the crisp alliteration of "called" and "construct" (bolstered by the consonance of "construct") draws readers' attention to the fact that the speaker here is acting as a creator (another alliterative word!): he's being "called in"—by whom, readers will likely wonder—to make up a religion, presumably out of thin air.
The next two stanzas feature alliteration as well, both using the heavy /d/ sound. In line 6, the speaker declares that the church-goers will cross through water and then change into "dry, different clothes." The alliteration here sounds firm and emphatic, highlighting the (literally) transformative power of this ritual: people will come to church wearing street clothes that will get wet during the "fording" and then change into something new (now being cleaner and symbolically purified). The thudding /d/ alliteration adds a subtle sense of gravity to the poem's description of this ritual.
Similarly, in line 9, the repeated /d/ sounds of "devout drench" seem to evoke the almost violent intensity of the "[i]mages of sousing" the speaker's liturgy would employ. And in the poem's final stanza, the quick alliteration of "water / Where" and "any-angled" close the poem on a musical note.
This alliteration might be a way of heightening the poem's language to reflect the speaker's earnest belief in the power of ritual. On the other hand, the exaggerated sounds may also subtly communicate the speaker's ironic attitude toward these rituals. That is, the speaker might be being purposefully dramatic, heightening the poem's language to reflect the idea that these practices are merely "construct[ed]," silly, and unmoving.