story identification - Novel about underwater city with separatists

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:
  
  
  
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    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.

    
    

    
    
  
  
  
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    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.

Answer

I'm not certain, but it could be Crisis on Conshelf Ten by Monica Hughes.

It involves a young man coming to an underwater community (however, he actually comes from the Moon, not Earth!), and gets to know a girl of the same age (the girl is dating his cousin, which might be similar enough that you'd think she's family):

Kepler Masterman has spent his entire life on the Moon, something that becomes a disadvantage when he visits Earth for the first time at the age of 15. Unaccustomed to the gravity, he has difficulty walking, and keeps passing out and having severe nose bleeds. His only relief is the hotel swimming pool, something that gives his father an idea: during this six month stay until a return rocket is available, Kepler should spend his time on one of the new colonies under the sea. It should be healthier and safer. Should.

Before heading down beneath the surface, Kepler receives some basic SCUBA and other underwater training from Hilary, a beautiful teenager from one of the underwater communities who is finishing college up on land.

There's a separatist movement, and genetically-engineered gillmen, and Kepler does get in trouble before he discovers this:

The underwater communities are different in another way as well: some of their members have been able to adapt completely to an underwater life, modifying their bodies to have gills instead of lungs. The unaltered humans call them gillmen and gillers; they are rare enough to remain a legend to some parts of the community, but most of the community is well aware of their existence—and not eager to share this secret with anyone from the land or the moon.

Kepler, naturally, finds out, after he does something stupid and needs rescue. Which leads him to the next discovery: the gillers and Hilary, increasingly desperate over policies, are turning to violence and sabotage in a desperate hope to change the status quo.

I can't find a specific reference to Hilary's brother being presumed dead (and I read it in the 80s so my own memory doesn't help), but it's possible.

(summary quotes are from this Tor article on the book).