Under the sea we've got a whole host of fishy creatures making cameo appearances. We have actual fish of course, as well as starfish, jellyfish, mussels, and some crabs. It's a regular Red Lobster buffet. But on a more symbolic level, all those creatures represent different life forms living together in one big aquatic ecosystem. They're kind of like us, interconnected and trying to get along in the big world.
Lines 1-2: First we have the title "fish" swimming through "black jade" waters, which tells us there's nothing easy about this aquatic life. All the figurative language makes the fish's movement seem more challenging than free.
Lines 3-4: Then those mussel-shells keep opening and closing like "injured fans" while resting atop "ash-heaps" that are presumably other dead life forms. So we begin to sense that these creatures represent more than just themselves. In fact, they look more human and vulnerable than anything else.
Lines 15-18: It's a "turquoise of bodies" at this point, but with all the ambiguity we sense that those sea creatures may be symbols of human "bodies," both dead and alive. We might also have an allusion to wartime here too, bearing in mind the time period.
Lines 20-25: Everybody's getting along and "sliding each on the other," so we get the sense that things are looking pretty peaceful at this point. Everybody is working together—good times.