This poem doesn't follow any formal guidelines. But Bishop does like to keep things tidy.
The poem is made up of six stanzas, all of six or seven lines each (except for the last stanza, which has eight). You'll notice the lines are all pretty much the same length too; there are no super-short lines, and no lines dragging to the end of the page. So when we look at the page, we get a sense of organization, even if there are no formal rules at work.
And yet. These patterns are worth examining…
Bishop keeps the lines relatively uniform and short. It's a very descriptive poem, with relatively little action, so perhaps she does this to keep the poem rolling, without getting bogged down with heavy, long lines that might slow things down too much. It also gives the poem a kind of listy feel, which reminds us that were reading details of description, rather than a narrative of some sort.
That's a fancy poetry term for a repeated word or phrase, especially at the beginning of a series of lines. Bishop uses "somebody" toward the end of the poem to establish a rhythmic pattern, and to get a little sound engine going to blast us toward the end of the poem. After about the second "somebody" our ears are perked up and we're paying very close attention to those final lines. She got us hooked.