This theme flashes by, but we think it's an important flash. Toward the end of "Diving into the Wreck," the speaker refers to herself or himself as a man and a woman. We often don't know the gender of the speaker of a poem. It's more unusual, though, for a poet to come out and directly confront us with that issue. This becomes one of the big turning points of the poem. Even though it's a little moment, it involves a lot of the other themes in the poem. It's certainly a big transformation, and it forces us to rethink the moments that came before it.
When the speaker's gender changes, it forces us to give up our assumptions about who he or she might be. Even if we have decided in our heads, we have to rethink that idea by the end of the poem. This makes gender a central subject of this poem, instead of an issue that the reader can overlook.