Where It All Goes Down
The two settings the speaker uses (the sea and the Titanic) are welded into one aquatic world in "The Convergence of the Twain." At first we're just checking out the solitude of the sea and the ship that's couched at the bottom. But then the speaker gets more specific and takes us way down to the bottom where all of the ship's useless things "lie lightless." So it's as if we get two settings in one. First the speaker gives us the big picture, then he zooms into the smaller details. It's kind of similar to what we'd see in a movie, The Lord of the Rings, for example. In the beginning we see some pretty landscapes, and then the director brings us into the Shire with tiny hobbits.
At the same time we're aware of an overarching "Immanent Will" that's driving everything together. For a moment we're traveling back in time to when the Titanic was being built under the influence of this immanent force and then we're reminded of its "later history." So even the time period moves back and forth, kind of like the ocean tide.
But the biggest players in the poem's setting are definitely the sea and Titanic. And once the Titanic becomes part of the ocean world with curious fish that can't make sense of it, we might even find ourselves contemplating the setting of humanity's truer self. In other words, the sea becomes a kind of setting that symbolizes man's inner world that's far from his ego and pride. So our speaker does more with the surroundings of the poem than just setting a stage for us.