I crawl like an insect down the ladder (line 30)
This little simile gives us an early tip-off to the theme of transformation. We don't think you are supposed to imagine the speaker turning into an actual insect, although that might be kind of fun – imagine a roach in a diving suit, little tentacles waving. By getting into the diving suit, the speaker has already begun to change. The suit changes this person's movement, makes them feel different, and that kicks off a series of transformations in this poem.
you breathe differently down here (line 51)
The speaker has to change in many kinds of ways to adapt to the underwater world. The first is to change clothes, to bundle up against the cold and the wet. Now even breathing has to change. It's part of a long transformation, as the diver stops being a single human and becomes a native of the sea.
the mermaid whose dark hair streams black, the merman in his armored body (lines 72-73)
Now the speaker has become a sea creature. Mermaids are literally a mix of the human and natural worlds. Rich turns those flippery diver's feet into an actual fin, but the hair remains long, flowing, and human. But there's another kind of transformation here too. The speaker is switching back and forth between male and female identities. He or she is a maid and a man all at once. The body is hard and male, the hair is long and female. The undersea change is wild, weird, and total. Right after this line, the speaker stops being "I" and becomes "we" almost like he or she had split in two.