The title, "Diving into the Wreck," lays out the basic idea here. To put it simply, the speaker is a diver, looking for the wreck of a ship beneath the ocean. She puts on scuba gear, and describes the process of descending to the wreck. First we hear about the trip down the ladder into the water, and then the big, scary, amazing feeling of entering the ocean. The sea and its life are all around, and then the speaker arrives at the wreck.
The speaker first flashes a light on the wreck, and then begins to explore, searching through the body of a ship. We go on a tour of the space, seeing the abandoned, waterlogged objects that fill the wreck. It's important to notice that the speaker has many more mysterious things to say about who she (or he?) is, and what this experience means. For one thing, she addresses books and words, and how they relate to the experience of the dive. This forces us to think about what it means to be reading a poem. All the same, the images of the undersea dive are laid out pretty clearly for us.