Fred, Fredericka, is a mermaid hybrid. Her father is a merman who washed up on shore and 'knocked up' Fred's hippie mom, Moon, and disappeared to the waves again. This left Moon to raise hybrid mermaid (and cranky) Fred on her own.
This being the second book in the series, Fred's story continues and she is invited to meet the Royal mer-family at a conference in the Cayman Islands. She takes vacation from her job as a marine biologist and brings her friends down with her.
She is walking the proverbial tightrope between the human and mer-folk sides, plus she is being pursued by two men, one mer-prince and a biped, as humans are called. This is all making her even more cranky.