The blackness of the Emperor Bone Sea really did remind one of the vast starry sky. The seemingly obvious vast emptiness was actually filled to the brim with life, forming a macrocosm in and of itself.
In the same vein, the deep sea seemed to mirror space itself as well. At this level, the obvious presence of water essentially vanished, the murky blackness in the surroundings instead beginning to feel like heavy space.
Water and space as elements generally mirrored each other more than most people realized. The spatial layers ebbed and flowed like tides, the heaviness of space felt like one was trying to walk while submerged in water. Space created a container, and water would expand however necessary to fill that container. The way they complimented each other was noteworthy when one realized the distinct similarities between them.