About twenty years ago, I read a sci-fi novel where, if I recall correctly:
- The protagonist is a guy, and probably a teenager or a young adult. Like most of humanity, he lives on land; but he's visiting an undersea colony (one of many?), and that's where most of the action takes place.
- He gets to know a girl there about the same age; maybe he's staying with her family for some reason? (Maybe they're relatives of his?)
- The girl had an older brother ("Ian", maybe?), but he died.
- He goes out of the city (i.e., goes for a dive) at some point, and gets in trouble, but is mysteriously rescued.
- He learns about an underground separatist movement of some sort. (I don't remember the details.)
- He eventually discovers that:
Ian is actually alive, and his family (or at least his sister) is still in touch with him; but they're covering up the fact that he's joined this separatist movement, and (like some other separatists) has modified his body to have gills so he can live out in the ocean.
Ian is actually who rescued him mysteriously.
I would guess, in retrospect, that the novel is from the 70s or so; but I don't really know.
I read the novel in English, and I don't think it was a translation.