Gillyweed

Gillyweed is a magical plant native to the Mediterranean Sea. When it is eaten by a witch or wizard, he/she grows gills and webbing between the fingers and toes. There is some debate among Herbologists as to the duration of the effects of Gillyweed on fresh water versus salt water, but the effects of Gillyweed in fresh water seem to last about an hour.
Gillyweed is said to resemble a bundle of slimy, grey-green rat tails.
The boy wizard, Harry Potter, used Gillyweed during the Second Task of the 1994 Triwizard Tournament.

Links:

Harry Potter and the Goblet of Fire
J.K. Rowling