Plain Jane and the mermaid

Jane knows she's plain because everyone has always told her that her whole life, but Jane's parents have passed on without a male heir, meaning she is about to lose her house and her fortune if she can't find someone to marry, so she comes up with a plan to propose to the handsome Peter, who might just say yes to get away from his father. But when Peter is kidnapped by a mermaid, Jane embarks on an underwater adventure to rescue her maybe-fianc?, where she encounters tricky water gods, beautiful and terrifying marine creatures, and a snarky seal that's more than he seems. Jane discovers that it's what's on the inside that counts, and she realizes that she's fine just the way she is.