Sealskin is a work in progress (not yet on itch, even in draft form, but about 75% of the manuscript is done).
It is 5e-compatible and presents rules for playing selkies. This includes a discussion of safety tools for handling what happens if a selkie's sealskin is stolen. Also includes less meta content: magic items and spells, an NPC gallery, random tables for fey crossings, nonsexual reasons why an NPC might steal a sealskin, etc.
If you want, you can check out bits I've posted here: https://glaucushauriant.blogspot.com/search/label/selkie