fiction
The book is an autobiography, so ... theoretically non-fiction, with likely some "enhancements" as is customary in autobiographies.
Non-fiction is a book with factual information. Some books, such as comic books or poetry are classified as non-fiction regardless.
Mermaids are a fiction topic. They were first introduced when Greek fishermen caught sight of dugongs, but mistook them for young maidens with the top half being human features, but the bottom half...