I gave it to the Longbottom boy, don't you remember? Magical Water Plants of the Mediterranean. It would have told you all you needed to know about gillyweed.
-- Barty Crouch Jr./The Fake Moody (GF35)
Magical Water Plants of the Mediterranean is a guide to magical marine plants.
The fake Moody gave Neville a copy of this book after the first Defence Against the Dark Arts lesson of Neville's fourth year; it discusses gillyweed (GF14)
Fake Moody (Barty Crouch Jr.) wanted Harry to win every task in the Triwizard Tournament in order to lead him to Voldemort, and he knew the Second Task involved diving underwater. So the plan was that Neville would read the book then suggest gillyweed as a solution. However, Harry never asked Neville for help, so fake Moody/ Barty Crouch Jr. made sure that Dobby the house-elf overheard a conversation about gillyweed between himself and Minerva McGonagall in the Staff Room. Always helpful, Dobby stole the gillyweed from Snape's potions closet and gave it to Harry (GF26, GF27, GF35). Snape was so upset that both gillyweed and Polyjuice Potions ingredients were missing, that he threatened to put Veritaserum in Harry's pumpkin juice (GF27). In the end, Dumbledore told Snape to give Veritaserum to the Fake Moody and the truth came out (GF35).