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The story is set in a submerged city or community complex, anchored to the sea floor by tethers I believe. The story is centered around the POV of a kid, probably teenage. He was portrayed maybe 14-17 years old.
The first event I remember is one of these anchored homes caving in and collapsing to the sea floor. The main character (MC) witnessed the event and rushed in to save and rescue what/whoever he could.
The second event I remember is the MC and some female friend of his exploring an abandoned wreck of some structure and, in the midst of it, hearing the sound of a distant whale outside the structure propagating and reverberating through the metal structure.
The third event I remember is the ending, however, I'm presently on the mobile app of SE and don't see a button for spoilers (despite the ample space available for one) and I don't remember the special string of characters needed to create a spoiler, so I'll save typing it for later. Suffice it to say that the story ended happily, with plenty of dolphins.
There are other aspects of the story I remember as well.
I believe the MC lived on an aquatic farm homestead sort of ordeal, growing some kind of aquatic plants for the community. Their nearest neighbors did this as well. They all lived some distance from a central community with hundreds or thousands of citizens involved.
Most of these civilians were in denial of "mutants" emerging from their specialized community. I believe they shared stories about witnessing the feats of these mutants like we landgoers share ghost stories. In the end the MC and a number of other children end up being mutants themselves, adapted in some minute ways for living within water.
Some of these hypothetical mutants are blamed for a string of incidents and disasters in the communities and city, such as that one homestead collapsing and sinking (also, as it comes to me, I believe they managed to patch the sunken house up and inflate it again). I believe that during that event the MC chases down a culprit and witnesses something or someone mutant-like, driving his own suspicions and the plot further.
To live and work in the water, I believe the citizens inhaled in a kind of oxygen-rich liquid which allowed them some time to function in the depths without special breathing apparatuses.
I don't remember much about the regional setting, like where or when this place was situated, but I think there was a coastline city nearby.
The story seemed geared toward young adults and was likely written recently. I'd be surprised if it was written any later than 1990, and my impressions tell me sometime between 2000 and 2010. It was written in English and the copy I borrowed was printed on a paperback.