When you've got a poem about fish, there's no escaping the significance of the sea. In Moore's poem, the sea is more complicated than we may initially think. On the one hand it's pretty and serene and on the other it's violent and old. We just can't make up our minds. So in a lot of ways the sea symbolizes all the different cycles and perceptions of life. Here are some key cameos that sum it all up:
Lines 1-2: The fish are wading through "black jade," which tells us the speaker's perception of the sea at this moment is rather cold, hard, and dark. The water here doesn't strike us as something easy and peaceful.
Lines 15-18: Here it's a "turquoise sea of bodies," which sounds a whole lot different from the cold and dark imagery of the previous stanzas. At this point we get a new perspective, one that's light and filled with life (though we're not sure what kind of life just yet, thanks to all the ambiguity going on).
Lines 18-20: Yet another shift in mood and perception occurs here with the water "driving an iron wedge" through the cliff. At this point, things are looking pretty violent and nothing like the peaceful sea we saw just seconds ago.
Line 40: If you think grandma is old, you better believe the sea is way older. Here it "grows old" in the cliff, suggesting that there's something ancient and enduring about the sea and nature.