Another idea is "hanging gardens." You could imagine forests of vine-like fungi dangling from a low ceiling. Passage through such areas would require small boats or "icebreaker" boats with large prows that sever/push the vines out of the way. This could be the habitat of albino hadozee or other subterranean simians.
A similar idea would be creatures that look like barnacle/worm hybrids. Imagine giant barnacles on the ceiling of the cavern. From their shells, they extend worm-like bodies down to the surface of the water, where they graze on nutrients brought by the underlying currents. There might be large clusters of such creatures above slow-draining whirlpools. leading to deeper caverns beneath the surface of the sea. All sorts of parasites could cling to or climb through the "forests" of barnacle-worms.
--Eric