It has a green cover with picture of a boy swimming next to a submarine. The fish farmer and his surface dwelling girlfriend go on an adventure.
The cover doesn't match up, but the plotline vaguely does for Dark Life.
In a future America where rising seas have claimed much of the land, leaving Topsiders to pack themselves in tiny stacked apartments, some people have taken to the ocean. These pioneers live on the ocean floor, farming the sea and sending crops to the government in exchange for their homesteads. This is where Ty Townsend has lived all his life. He was the first child to be born in the undersea community of Benthic Territory and he loves it there. But when outlaws threaten the safety of the settlers, Ty must do what he can to save the only home he's ever known. And when a Topsider girl appears, searching for her prospector brother, she'll join him on his quest and together they'll find that there are secrets being kept about the Dark Life.
Some of the other reviews mention two submarines. Gemma is found in one by Ty. And another is shark-shaped.