I saw this wreck as a situation she was told about and has been thinking about for some time. She also felt that she has been preparing herself for it in the first stanza (see pg 1826). The "knife blade" can be seen as the edge she felt she had in the coming situation. As she progresses through the wreck, she finds that she is forgeting why she decided to explore the wreck. But forces herself to focus on what she can see. She is not down there to make up more stories or discover myths. And in her final stanza she explains how everything and everyone in that wreck will be there for a long time but the passing explorers and divers will never be remembered or kept track of. The courage and cowardice she speaks of, are the reasons for why people even decide to go to such depths to see something. In other words, some people wish to understand something out of fear while others want to dive into the situation head on.
