Chapter 18 begins with Lina, Doon, and Poppy continuing their travels after leaving the Pipeworks behind.
They row their boat towards the mouth of a tunnel. For a time, the river current is tempestuous, and the three travel in silent fear. After a while, the waves calm, allowing Lina and Doon to light candles. With light from the candles, they are better able to see where their boat is headed.
Soon, they come to the mouth of the tunnel, which opens up into a cavernous space. There, columns of rock hang down from the cave's vaulted ceiling and rise up from the waters. The columns of rock glimmer in colors of green, silver, and pink. Lina and Doon are mesmerized by the sight as they row along.
After a time, the current grows rough again. The wild splashing of water puts out the candles, and once more, the three must travel in darkness. As for Lina, she loses track of time.
Eventually, they come to a pool—this is the final pool your question references. The text tells us that Lina has no idea when the current slowed and how long (whether a few minutes or an hour) it has taken them to reach the pool.
However, she does notice that the ceiling is only ten feet above their heads. In addition, there are no columns of rock here. Eventually, Lina and Doon stop rowing at the edge of the pool. They get out and begin surveying the scene before them. The way forward doesn't look promising.
While Doon stays with Poppy, Lina gets out to explore. Here and there, the path seems narrow. Eventually, however, she discovers a smooth path. Excitedly, she tells Doon about it. To their joy, the path is the right way forward.