As the poem begins, the speaker remembers finding a grim "relic" on the seashore: a fish's picked-clean jawbone thrown up on the sand by the waves. This jawbone will become this poem's central symbol, an image of nature's indifferent brutality. In this speaker's eyes, life in the sea—and, when you really think about it, life anywhere in the natural world—is nasty, brutish, and short.
To set the scene, the speaker depicts the beach as something like a battlefield. The ocean, the poem observes, is constantly flinging casualties ashore. Take a look at the meaningful repetition here:
There, crabs, dogfish, broken by the breakers or tossed
To flap for half an hour [...]
"Broken" and "breakers" aren't just an instance of polyptoton; they also introduce a pun. On the surface, the speaker means "broken" in the sense of "injured" and "breakers" in the sense of "waves" (said to "break" on the shore). But those "breakers" also seem to be, well, breakers here, breaking those crabs and fish apart. The ocean itself, the poem suggests, smashes its own creatures to bits.
Brutal crushing isn't all the ocean does. Even as these dead, mangled animals decay and "turn to a crust," they "continue the beginning." Their deaths, in other words, aren't just a grisly ending. They're also the start of something new. And if they "continue" the beginning, then that "beginning" goes on and on. Exactly how this might be true will be one of the poem's big themes.