For this passage, I like the adjective sinking. It looks like a verb, but can be used as a participle which is an adjective. Here is why I would choose it.
From the beginning of the passage to the end, I get the feeling of going from what it is like on top of water, to what it feels like completely submerged by water. Perhaps this sinking could actually be a metaphor for what Ralph is feeling at the moment. At this point in the book, I find the boys sinking further and further into their savage human nature, and the further they "fall" the further they travel away from possible return to the life-giving air above.
The language begins with references to air, and towards the middle it uses the "swell" and "breathing" like waves feel when close to the surface. Then, "down, down, the waters went". Finally, the idea of an ocean floor surfaces.
There may be MANY adjectives that can describe this passage, it just might be worth working through the passage to see if you can find text to support it.