Free verse is a term used to describe a poem that doesn't have a regular rhythm or rhyme structure. That doesn't mean that there isn't any structure at all, but that the structure can't be put in one particular category. That gives us a chance to look for more subtle patterns. For example, the first line of this poem is written in perfect iambic tetrameter. Even if we don't see that right away, or we can't remember the name of that kind of meter, it still changes the feel of the opening. It gives it a smooth, steady quality, and it links up with the idea of a "book of myths" because it feels a bit like the opening of an old story.
This poem is carefully structured in other ways. It's split up into ten stanzas, all of which are more or less the same length (eight to twelve lines). When there's a break between stanzas in this poem, it signals an important shift. Sometimes that means a move from one place to another, like from the boat to the ocean. Sometimes it's a change in perspective.
Also, check out how chopped-up these lines are. Often a whole stanza will be just one sentence, split into a number of shorter lines. When a sentence continues after a break in a line, it's called "enjambment." For example, the pauses created by the enjambment of the lines, "Rung after rung and still / the oxygen immerses me" (23-24) give the impression that the speaker is preceding very cautiously down the ship's ladder. That's one way Rich sets the rhythm of the poem, giving it a stuttery quality that changes our experience.