There is a ladder. The ladder is always there hanging innocently close to the side of the schooner. We know what it is for, we who have used it. Otherwise it is a piece of maritime floss some sundry equipment.
Interestingly, these lines are all about one thing: a ladder.
You've probably seen a ladder like this before.
It hangs off the side or the back of a boat, and dips into the water. For a diver, it's a way to get into the water.
This poem emphasizes the separation between air and water, between the space above the water and the world below it.
That's why this ladder matters in this poem. It's what allows you to cross between two worlds, to move from air to water.
Rich takes her time here, pausing to think about this ladder, to make us look at it.
What fascinates her is that the ladder has a different meaning once you've used it.
For people who aren't divers, that ladder "hangs there innocently." It's nothing special; you could walk right by it. Or as she puts it, it's "a piece of maritime floss, some sundry equipment" (20-21).
("Maritime floss" means something like "a little string from the sea," and "sundry" just means "random" or "miscellaneous.")
Both of these phrases are meant to emphasize how ordinary this ladder is.
But once you have been on a dive, you know that going down the ladder is a major moment. We've probably all had this experience in some way.
Before you drive a car, you don't really know or care what the different parts are, but once you do, you realize how important a gearshift is, why a brake pedal could mean the difference between life and death.
Relying on a thing, even an ordinary thing, brings it to life and makes it special.
I go down. Rung after rung and still the oxygen immerses me the blue light the clear atoms of our human air. I go down. My flippers cripple me, I crawl like an insect down the ladder and there is no one to tell me when the ocean will begin.
Now we hear about the speaker's experience descending the ladder.
For now, the speaker is still in familiar territory: "the oxygen immerses me" (24).
We usually use the word "immerse" to talk about a liquid, when we submerge something. So this word choice makes it seem like the speaker is swimming in the air.
We can feel a sense of safety, of normalness in "the clear atoms of our human air" (26-7). At first it seems like an obvious point, but our speaker wants to emphasize how we are dependent on the air.
Since humans must breathe air in order to survive, it isn't normal for humans to enter the sea, to become part of that world.
Even the equipment is unnatural; the flippers "cripple" (29) the speaker and make her "crawl like an insect" (30).
This process of changing worlds is hard, awkward, and maybe a little scary, especially when you are alone.
The speaker emphasizes loneliness again, reminding us that "there is no one to tell me when the ocean will begin" (32-3).