Poisoned Water (sometimes Toxic Water) is a poisonous liquid found in some games. It is commonly purple or dark green, and in most appearances can kill Mario or any other member of his party instantly.
Super Mario series
Super Mario Sunshine
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.
New Super Mario Bros./New Super Mario Bros. Wii
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 kills Mario and Luigi instantly. Dorrie, Koopa Troopa, and Giant Wigglers are resistant to Poisoned Water.
Super Mario Galaxy
In Super Mario Galaxy, an enormous planet made of poison liquid that can kill Mario instantly 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.
Super Mario 3D Land
In Super Mario 3D Land, World 2-2 and Special 6-3, both underground levels, contain Poisoned Water, which is purple once again.
New Super Mario Bros. 2
New Super Mario Bros. U
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.
In Wario World, poisonous water appears in Unithorn's Lair. In this game, it will not damage Wario, but rather cause him to bounce around the room and lose coins while doing so. The poisonous water appears mostly translucent, but with a shining or bubbling surface.