The Girl Who Pulls the Tides - ONC 2021

There's an unspoken rule on the island of Norholm, whose shores are haunted by sea nymphs that dwell deep beneath the ocean and only surface twice a year when the moon glows blue. The rule is: don't bother the sea spirits, and they won't bother you.
Fionn Miller is not the best at following rules.
Especially when there's a mystery to be uncovered.

This story is being written for the Open Novella Contest, inspired by prompt #23.