4 ideas
After his lover, the daughter of a yakuza boss, dies of the cancer, he couples with one of the mermaids and she bears a human-mermaid hybrid. The rest of the book follows the fallout of a child that should not exist under the rules that make mermaids property.
Mermaids are kept in the legal and cultural category of animal, which lets humans kill, eat and sexually use them without it counting as a moral violation. The real taboo is not harming them but breeding with them. A hybrid offspring would prove the boundary is arbitrary and threaten the economy built on it.
When a sun-driven epidemic makes survival scarce and forces humanity undersea, social order does not reset toward solidarity. Criminal syndicates take control of the scarce goods, food and pleasure, because they already run supply chains outside the law. Collapse raises the value of whoever controls the flesh trade, so existing hierarchies harden.
The farmed mermaid is sold both as meat and as a sexual object, so eating and desire run through the same market and the same creature. This fusion shows how consumption can swallow intimacy: the body wanted most is the one most completely used up. Desire becomes indistinguishable from devouring.