The Drowned Heir

When her uncle dies at sea, a third child with no place in society undergoes a ceremony to house his spirit and take his role, diminishing the family's loss. But her uncle's spirit hasn't settled the way it's supposed to, and will not content itself with shore-bound business. Her uncle's spirit insists, angrily, that it was not just a storm that killed him and wrecked his ship, not a rogue wave; it was an unthinkably large monster. 

Then his lover comes knocking with news of an adult son who has set sail along the same shipping lane, and dead uncle and living niece must work together to save a son neither of them knew existed.

[this novella is ~20,000 words]