Gillyweed
A magical plant native to the Mediterranean Sea, Gillyweed could be eaten raw resulting in the appearance of gills on the neck, webbed hands and feet and the ability to breathe underwater. Provided by Dobby (or Neville in the films), Harry Potter utilised Gillyweed's magical properties in the Triwizard Tournament when one task demanded that he dive into the depths of the Great Lake in order to save Ron Weasley from the bottom.
