Poisoned Water (sometimes Toxic Water) is a poisonous liquid found in some games. It's commonly purple or dark green, and in most appearances can slay Mario instantly.
Super Mario series
In Super Mario Sunshine, Poisoned Water appears in a secret area found within Delfino Plaza. In this area, Mario must ride on a leaf down the Toxic Water in order to collect eight Red Coins. Also worth noting is that in certain episodes of Bianco Hills, Ricco Harbor, and Noki Bay, the water is polluted and can still damage Mario, but cannot instantly kill him.
In New Super Mario Bros. and New Super Mario Bros. Wii, Poisoned Water can be found throughout several stages, mostly in the forest worlds World 4 from the DS game and World 5 from the Wii game. In these games, it's purple and kill Mario instantly. Dorrie and Giant Wigglers are the only characters that are resistant to poison water.
In Super Mario Galaxy, an enormous planet made of poison liquid is found in the Bubble Breeze Galaxy. It is also found in the Luigi planet from Toy Time Galaxy. An element similar to poison called Dark Matter appears in Bowser's Dark Matter Plant. In the sequel, the toxic water is found in Boo Moon Galaxy (in the Swamp Planet) and in Mario Squared Galaxy as well.
In Super Mario 3D Land, World 2-2 and Special 6-3, both underground levels, contain Poisoned Water, which is purple once again.
Donkey Kong series
Donkey Kong Country
Donkey Kong Country 2: Diddy's Kong Quest
Donkey Kong Country 3: Dixie Kong's Double Trouble
In Donkey Kong Country 3: Dixie Kong's Double Trouble, Poisonous Water appears in Poisonous Pipeline. This time, the poison inverts the controls to move Dixie Kong and Kiddy Kong. Jumping out of the poison, the apes instantly go back to normal controls.