Gillywater

Gillyweed is a supposedly "magical" plant that, when consumed by a human, is said to give them fish-like attributes, including gills to process oxygen from water, webbing between the fingers and toes for easier swimming, removing the need for blinking, and adapting to cold temperatures in water. The plant itself is said to resemble a bundle of slimy, grey-green rat tails and is native to the Mediterranean Sea.