|"Is this all real? Or has this been happening inside my head?"
The subject of this article is a real-life person, company, product, or creative work that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
Flora and fauna
Magical creatures which lived in the Mediterranean Sea included the hippocampus, the sea serpent, and merpeople. The merpeople found off Greece were the first to be mentioned in historical records and were called sirens. They, along with merpeople from other warm-water regions, were noted for their physical beauty, and inspired the popular depiction of the species in Muggle art and literature.
Magical Water Plants of the Mediterranean was a book that covered the sea's native flora. One species it discussed was gillyweed, which caused a person to grow gills and webbing between their feet when eaten, allowing them to breath underwater and swim more easily.