Piranha Water is a damaging water found in Bubblegloop Swamp and the area around its entrance. While Banjo and Kazooie can run through the shallow water, they will constantly get bitten by piranhas every few seconds. Piranha water can be safely crossed with the Wading Boots, the Wonderwing, or the Crocodile transformation. When Banjo and Kazooie step into it for the first time, a piranha will say "This is piranha water, keep your smelly feet out!".
Banjo-Kazooie Wiki