Nibbles

Nibbles (also called Piranha) are Chomp-like fish that inhabit the waters of Rogueport and its surrounding regions in Paper Mario: The Thousand-Year Door. If Mario falls in almost any body of water, a Nibbles will bite him, causing him to jump back to the same place he fell into the water. He will also lose one HP. These enemies cannot be battled. A few Nibbles can be seen in Sir Grodus's room in the X-Naut Fortress, where they are in an aquarium surrounding most of the room. Nibbles are even found in swimming pools and fountains. One appears in the lower right-hand corner of the Magical Map.

Trivia

 * Goombella's tattles imply that the plural of "Nibbles" is just "Nibbles".
 * Between Chapters 4 and 5 in Paper Mario: The Thousand-Year Door, Bowser falls into the sea as Mario does, but no Nibbles appears and bites him.