Undead and Underwater

This story is a crossover with the "Fred the Mermaid" series by the same author. According to the story notes, this takes place after Undead and Unstable. "Betsy, queen of the undead, and Fred, the dour could've-been-a-princess mermaid, reluctantly team up to help a mutual friend in trouble. After all, a friend in need is a friend who, well, can really test your patience—because Betsy and Fred aren't exactly BFFs. What they do have in common is the source of the problem: a man. And what's more troubling than love?" Synopsis from the back of the Undead and Underwater collection.