Are you sick of alliteration yet? Of course you're not Shmoopers, since it works pretty well in "The Convergence of the Twain." Plus it has a purpose, other than just looking cool in print. In Hardy's poem, alliteration helps to accent things like those jewels that are "bleared, black, and blind," (12) since they don't have any light to help them sparkle. The repetition of similar sounds adds to that contrast between opposites that the speaker loves to point out for us. Notice how examples of alliteration, like the one above, are usually juxtaposed with man's vanity, whether we're talking about jewels or "smart ship[s]" that clearly aren't that smart.
Even the "solitude of the sea" (1) reminds us of just how lonely it is out there in the middle of the ocean without cell phones and trinkets to keep us warm. All we have is the soft splash of the waves—that, and who we are without all of those "things." And since the Titanic was largely about luxury, it seems the speaker is trying to convince us that trinkets amount to absolutely nothing in the end. So moments of alliteration serves two purposes in this poem: to keep things sounding smooth and to direct our attention to the differences between man's vanity and nature's reality.