Murder Water

DEATH WATER!

—Proton Jon

When Hazardous Water indirectly trying to kill you isn't good enough, it's time to take this to the extreme: The water itself has somehow developed a mind of its own. Current desire? Kill the meatbags.

For obvious reasons, actual water trying to murder someone is a little unlikely, so a common workaround is that the water is being possessed or manipulated by a malevolent force—in which case, the only way to stop it is by taking out whomever does the controlling. (This doesn't always mean killing them; sometimes the heroes can reach an agreement, such as doing a favor or request in return for stopping the water's deadly rampage.)

Examples of Murder Water include:

Anime and Manga

Sloth from the first Fullmetal Alchemist anime.
In Naruto, Suigetsu can not only turn himself into water, but he can control and shape up to a lake's worth at will. Kisame has a variation on this trope in that he creates the water from his chakra, allowing him to flood areas and drown people wherever he is.

Film

The sequence in the Prince Caspian movie where the bridge is destroyed (though the water-god was on the good side).
Another morality-flip occurs in The Fellowship of the Ring, with Gandalf and Elrond (book) or Arwen (movie) summoning the crazy death river horses.
At one point in The Abyss the aliens decided to exterminate humans, and created tsunamis so huge that, if they hadn't been persuaded to change their minds, the waves would have met in Oklahoma.
The Raft segment on Creepshow 2 is about a malevolent oil slick in malevolent water. "I beat you!"
In the first Final Destination movie, the toilet that Tod has just used leaks a puddle of water that actively follows him until he slips on it and strangles himself on the clothesline. Then it retreats quietly unto the crack under the toiler.

Literature

In Lady Friday by Garth Nix, if you're not wearing the proper clothing (i.e., made of paper and letters), the water in the Middle House's Canal will actively try to drown you.
A Mercedes Lackey novel, The Outstretched Shadow, had a water golem sent after The Obi-Wan, who was on the docks at the time looking to escape the City. Of course, it looked like he had simply drowned.
In The Tales of Alvin Maker, the Big Bad (the Unmaker) primarily acts through possessing water for some reason. Apart from simply drowning people, at one point it froze the water, causing a knot to form in a particular tree, causing a weak point in a particular log, causing a building to collapse on the hero.
Again, The Lord of the Rings, when Gandalf and Elrond cause a flood to wipe out the Ringwraiths chasing Frodo and company.
In Robert E. Howard's Conan the Barbarian story "The Pool Of The Black Ones", the water will shrink you into a figurine. It's not nice even without that.

Live-Action TV

Doctor Who: "The Waters of Mars" had an ancient contagion released from beneath the surface of Mars that was so virulent that just one single drop would infect a victim completely. Oh, and turn them into a walking bloated terrifying zombie.
An non-lethal example appeared in an episode of Round the Twist, which featured a vengeful water spirit using her water-controlling abilities against those she perceived as having damaged her stream by building a dam in it.
In the X Files episode Aqua Mala has this in the most literal form.
In an episode of Supernatural, the ghost of murdered boy is possessing the water in a town and drowning the families of the men responsible for his death. Everything with water, including the kitchen sink, is a potential threat.

Music

William, the protagonist of the concept album "The Hazards of Love" by The Decemberists, pledges his life to a wild, magical river in return for allowing him to cross safely. After rescuing his beloved, William and Margaret attempt to cross back, but the river demands that the debt be repaid.

Video Games

The Legend of Zelda: Ocarina of Time had a boss that is effectively a pool of water. It has a round ball as a weak point and is subtitled as a "Giant Aquatic Ameoba", though it's unclear if only the ball is the boss and it's hydrokinetic, or if the whole pool is the boss and the ball is just its nucleus. After it's defeated, the entire room dries up with astonishing quickness, supporting the latter idea.
Psychonauts has Raz's entire bloodline cursed by another psychic family to die in water, which forms a ghastly arm when Raz is near to drag him to his doom.
Brutal Legend has the Sea of Black Tears, which is Hazardous Water taken to its logical extreme. If someone is thrown into it, they are dragged underwater by what appears to be the sea itself to join the ranks of the world's Goth faction, called the Drowning Doom.
- both Brutal Legend and Psychonauts were written by Tim Schafer. One wonders if he can swim...
Sonic Adventure has Chaos, the God of Destruction, a Chao mutated by Chaos Energy with water-based skills.

Web Original

Although he's normally given connection to trees, the Slender Man is often thought of to have some connection to water; likely due to his fluid form (see: tentacles, variable height).
EAT, of The Fear Mythos. As soon as you touch the water, you're probably gonna get grabbed by tentacles. As soon as you get a drop of that water in you, you're absorbed into the great and almighty Hive Mind.

Western Animation

The Secret Saturdays had an episode with a microscopic cryptid that could turn the water it inhabited into a rather unpleasant collective monster.
The dark water in The Pirates of Dark Water consumed anything that touched it. The main quest was to find the 13 artifacts required to neutralize it.
One episode of Challenge of the Superfriends has members of the Legion of Doom, using a helmet that turns dreams into reality, transform the ocean into an anti-Superfriends weapon that, surprisingly, the Superfriends can't figure out how to defeat. Don't worry, though: Despite having possession of an apparently invincible superweapon, the Legion managed to find a way to screw it up.
As a hydrokinetic, Liquidator of Darkwing Duck was easily able to do this. However, in the comic revival, a strange force (Other than Liquidator.) has taken over the entire bay in St. Canard, pulling people into it and seemingly regurgitating all the negative emotions of the citizens as tidal waves and currents. Turns out Paddywhack, a One-Scene Wonder from the original show, is possessing the bay.
The irate tenant of the Fantastic Four mentioned that water from her sink once attacked her.
A two-part G.I. Joe episode featured a plot to make Shipwreck believe he'd just woken from a decade long coma as part of a plan to obtain the missing ingredient from a chemical formula that turned water into a weapon grade explosive.