Titles are very important to Moore's poems. We see her using the title of "The Fish" in a way that is directly connected to the poem's body. In fact, without it, we wouldn't really know the subject of what's "wad[ing] through black jade." But that lower case "w" in the first line alerts us to the fact that the poem actually begins with the title, literally.
And when we think about some of the life-stuff questions the poem asks, we may find the title even more fitting. There's not much in the way of overt religion in this poem (though you could argue that any writing that tackles life and death and meaning has a connection to religion). All the same, by throwing out a well-known symbol of Christianity in her title, Moore is certainly calling to mind a big, cosmic picture before the poem even gets going.
But beyond clarifying the poem's subject, the title also sets us up for that symmetrically fishy form that we get. The poem actually looks and sounds like a fish wading through black jade with those funny, but perfectly-ordered, lines. So it makes sense that Moore would use a title that focuses our attention on the body and image of a fish.
Take a look at some of these other Moore poems (here and here) that also work with their titles in a direct way. You'll notice she does this a lot.