"Gillyweed is a magical plant native to the Mediterranean Sea. When it  is eaten by a witch or wizard, one 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 seems to last about an hour.  "

"Gillyweed is a magical plant native to the Mediterranean Sea. When it is eaten by a witch or wizard, one 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 seems to last about an hour. "