Isso pode ser Anjo com a Espada por CJ Cherryh - o primeiro romance do Noites de Merovingen série - publicado em 1985. Trama da Fantastic Fiction resumo corresponde a muitos dos detalhes que você mencionou:
In Merovingen, a watery canal-laced city, much like Earth's Venice, society is segregated along class lines between the lower and upper cities. Against her better judgment, Altair Jones, 17, rescues an unconscious man from a canal near her poleboat. She is fascinated by Mondragon's good looks and elegant ways and begins to fall in love with him. Even though she knows there is no future for a water rat like herself with such a man, she decides to watch over him and rescue him from his enemies; enemies who turn out to be the most powerful people in the upper city.