My daughter needs a book title, anybody?

My daughter wants to find a book she’s read before. After lots of brain pounding, she asked me to seek help from the jellies. She forgot the title.
The world has become divided into ocean dwellers and land lubbers.
There’s an adventurous young man of the ocean dwellers who likes to visit a wrecked sub. During a visit he finds a girl who presents herself as a lubber, but really she goes goes both ways. Ahem.
The girl is searching for her brother. When she tells this boy, he decides to join the search.
The boy’s father is involved with a group who is hunting down a gang of house ransackers.
Things get dark, and the plot turns a bit.
Boy ends up girl’s hero.
Babyburger thinks their age is about eleven or twelve.