The poem opens with a list of the speaker's preparations for the dive "into the wreck." In preparation the speaker has "read the book of myths," which immediately signals that the poem is probably not mainly concerned with actual diving.
The word "myths" is important. Myths are stories, often those that explain the origin of something (think of creation myths, for example). To say that something is a myth also means that it's not based in fact, and instead is a false belief. As such, this "book of myths" would be a book of familiar stories about the wreck—but stories that nonetheless aren't necessarily the truth.
The "wreck" itself in turn might represent any number of things. Perhaps this "wreck" refers, metaphorically, to some trauma in the speaker's past (on that note, many critics read the poem as having been influenced by the earlier disintegration of Rich's marriage and death of her ex-husband by suicide). Or perhaps the "wreck" is a metaphor for the oppression of women throughout history.
The specifics are deliberately ambiguous and open to interpretation. What's important is that, if the wreck is read as a metaphor for the past (the speaker's, society's, or otherwise), then the poem is already signalling a need to understand that past in order to go beyond it. The speaker seems concerned with a kind of truth-telling detective work, absorbing the stories of the past as part of the project to better understand the wreck—that is, to better some traumatic incident from the past that has gone on to shape the present.
The speaker has done other things to prepare too, such as loading a camera. This camera perhaps symbolizes an intention to record and catalog the truth the speaker finds, while the knife-blade and "body-armor of black rubber" suggest that there is something inherently dangerous about the journey the speaker is planning to undertake. Maybe these dangers are personal—the speaker will have to grapple with inner demons to face the truth of some traumatic incident—or maybe they're societal; if the poem is taken as a metaphor for women's liberation, then perhaps the weapon here indicates the way that challenging societal norms can be a perilous task.
Lines 4-7 are then a little more conventional in terms of diving equipment—black rubber suits, flippers, and masks are all important tools in actual diving. The anaphora (with the repetition of "the") shows the methodical way in which the speaker goes through each stage of the preparation, while the enjambment suddenly speeds up the lines:
I put on
the body-armor of black rubber
the absurd flippers
the grave ...
This speed suggests the speaker's eagerness to get on with these more mundane preparations, which apparently make the speaker feel a bit silly. The speaker says these flippers are "absurd" and the mask is "grave and awkward," reinforcing the difficulty of the task ahead—but also the speaker's discomfort; after all, most people look a bit ridiculous in full scuba gear!