It is important to remember that this poem is a loosely veiled journey into the subconscious where the speaker seeks to understand who they are and in particular how they relate to the "myths" that have inspired her to make this journey. The very first line makes reference to "having read the book of myths," and this is something that causes the speaker to begin her dive. Note the full reference to the quote that is included with this question:
the thing I came for:
the wreck and not the story of the wreck
the thing itself and not the myth.
In seeking to understand the myths and also the place of women in them and her own identity, the speaker realises that she must experience the wreck without the help or the guidance of books and language. It is necessary for her to experience the wreck by herself and to come to her own conclusions, rather than let mediated experience and ideas be her guide. The quote highlighted in this question therefore points to an objective reality that is beyond the ideas presented to humans in books and through other people; it suggests that personal experience is the most powerful and most important thing.