From the Super Mario Wiki, the Mario encyclopedia
Punch Piranhas are a species of shoaling predator fish found in Donkey Kong Country: Tropical Freeze. They live within the punch-filled pool over which the battle against Bashmaster in Juicy Jungle takes place. Such fish can be first seen in the boss fight's introduction, where they notice Bashmaster's popsicle falling in the lake and then voraciously assault it in groups.
Following the scene, the Kongs become able to interact with Punch Piranhas, which now serve the role of obstacles. Should the Kongs drop into the lake, a group of Punch Piranhas will start jumping out in that spot and hurl the characters back on the battle arena, taking away a heart from their health meter in the process. This prohibits swimming in the lake, or otherwise leaving the fight with Bashmaster. Likewise, Punch Piranhas cannot be destroyed.
It is implied by two dioramas in the game that, aside from Juicy Jungle, Punch Piranhas also live in the waters of Sea Breeze Cove and Donkey Kong Island, although they are not encountered there during gameplay. In the Sea Breeze Cove diorama, Diddy Kong catches a Punch Piranha using a fishing rod. At the bottom of the Donkey Kong Island diorama, a school of Punch Piranhas can be seen frozen in ice, along with Mugly.