Muggles' Guide to Harry Potter/Magic/Gillyweed

Overview
Gillyweed is a magical plant, apparently native to the Mediterranean Sea. It is described as looking like "slimy grayish green rat tails." Consumption of the raw weed results in gills appearing in the subject's neck, and apparently also hands and feet lengthening and growing webs to become effective fins.

Extended Description
When Harry Potter is having difficulty figuring out a way to complete the second task of the Triwizard Tournament (which requires him to rescue Ron from the bottom of a lake), Dobby steals some Gillyweed from Professor Snape's store cupboard and gives it to Harry to use.

We later learn that Professor Moody had given a book on Magical Water Plants of the Mediterranean to Neville, with the expectation that Harry would ask Neville for help in the Challenge. Harry never consulted Neville, however; so Moody deliberately mentioned Gillyweed in Dobby's hearing.

Analysis
More than breathing underwater, this plant forces partial adaptation to an underwater existence, providing Harry with gills and extending his feet and hands into flippers. These effects last for about one hour in fresh water.

Note: In the film version of this series, it is in fact Neville who provides Gillyweed to Harry; this is managed by having Mad Eye Moody summon Ron and Hermione to Professor McGonagall's office, when he confronts the trio in the library. He then introduces Neville, who mentions the plant while helping Harry put books away, instead of having Fred and George do it. This is likely a cost-cutting decision by the filmmakers; by eliminating Dobby from this one scene, they remove any need to have house-elves in the film.