Watering Down

In ten years in this dump, I've come to accept a very low standard of life. But even at the bottom of this fish tank, a man must have standards. The pathetic trou du cul down at Worley Winery has started putting water to his terrible wine... water!!!

—Pierre Gobbi, BioShock (series)

Diluting an alcoholic beverage (or some other drug), usually with water for alcohol, but sometimes other substances, is fairly common in both the real world and fiction. Street drugs are also almost always cut with other stuff. It is usually done so as to get more product from smaller amounts of genuine ingredients, yet sometimes used for other purposes.

Comic Books

In Preacher (Comic Book), Custer starts off his massive "The Reason You Suck" Speech to the whole town by telling the bartender he can almost taste the beer through all the water.
In some versions of the Batman franchise, the Penguin has shifted from outright crime to running a nightclub, where he makes far more money selling overpriced souvenirs and, yes, watered-down drinks.

Film

In American Psycho, a couple of the yuppies complain about how the cocaine they've been sold is "a gram of fucking Nutrasweet".
In the film version of Sir Henry at Rawlinson End, the ghost of the late Rawlinson Bulldog Gums takes possesssion of his now-stuffed body, and urinates through the floor of the upstairs corridor into Sir Henry's drink in the dining hall below. Sir Henry is horrified — the stuffed dog's bladder had been refilled with water. This watering-down is what prompts Henry to finally exorcise the ghost of his brother who put the dog up to it.

Literature

Examples from Discworld:
- A certain class of upper-class snobs like to label their alcohol backwards (so that whiskey is read as "yeksihw" and port as "trop") to prevent their servants from stealing sips from the bottles. In response, the butlers top the bottles up with "eniru" after they've had their fill
- Slide is a troll drug that's even worse than Slab, as it involves any junk lying around and requires pigeon droppings. And Discworld pigeons are described as "effluents on wings"...
- But a suggestion that a landlord has been watering down the scumble is quickly discarded, because everyone knows "what happens if you lets a drop of water touch scumble".
The book M.Y.T.H. Inc. In Action features Mob enforcer Guido discussing this phenomena, noting it's a way for the bar owner to make more money off less product, and his customers don't mind because less alcohol per glass makes the drink "healthier".
One of the minor characters in the Philip K. Dick novel Eye in the Sky is a hostess at a club who waters down her own alcoholic drinks (as a large amount of her job is drinking with customers) so as to not get drunk herself.
Lana Lee, the business-minded owner of the Night of Joy bar in A Confederacy of Dunces also waters down the drinks of her "hostess" Darlene...but she also waters down all the other drinks as a cost-cutting measure. Burma Jones frequently comments on it.

Live Action TV

Due to its heavy focus on drug gangs, The Wire features the drug version of this trope in spades. (Eg. numbers are thrown around between the gangs to talk about the strength of their product; 'Take it to ten' or 'This stuff is ninety', referring to what percentage of the product is actually the drug), and in hard times, they weaken their product by cutting it with whatever similar-looking substance comes to hand to make more profit. In season two, there are five deaths and eight hospitalizations in the Correctional Facility because the supply of heroin has been cut with rat poison.
Granny, a moonshiner, from Beverly Hillbillies, usually make strong liquor. But she usually waters it down for the city folks who couldn't handle it at full strength.
Jesse Pinkman in Breaking Bad originally spiked his meth with chili powder. Then he joined up with Walter White, and he put a stop to that.

"Chili powder. Did I not tell you how moronic that was?"

Rosie does this in MASH to keep the prices down.

Theatre

One of the things Monsieur Thénardier says he does in his Villain Song in Les Misérables is watering down the wine.

Video Games

In BioShock (series), minor character Pierre Gobbi has an audio diary complaining about the hellhole Rapture is turning out to be; but there is one thing he cannot stand: Worley Winery, producers of the fine wine Arcadia Merlot, watering down their wine. They then claim that at least they are using distilled water, and not seawater.

Western Animation

In the early The Simpsons episode The Crepes of Wrath, the French winemakers dilute their wine with antifreeze, then test it on Bart.

Real Life

Truth in Television to a massive degree. While alcohol with a little bit of water is often forgivable, Scare'Em Straight Drugs Are Bad presentations like pointing out how dealers cut marijuana and other drugs with everything from carpet strands to hair. Even camel shit. One would think that dried plants (you know, the thing pot is) would be more plausible, but then, plausibility doesn't scare anyone. Ironically, this is exactly why it's legal in the Netherlands. The government can't do quality control on illegal stuff, after all.
As mentioned by one of the examples above "hostesses" and "hosts" who work in bars or nightclubs to draw in patrons almost always have their own drinks severely watered down to allow them to not get too soused given the job requires them to drink along with the guests.
During Prohibition in the United States, illegal alcohol was often cut with methyl alcohol, a poison that can cause permanent blindness.
- This was also done DELIBERATELY by the government to prevent industrial alcohol from getting consumed. This worked as well as you might expect: not. Contemporary comments to this were: "The government is under no obligation to furnish the people with alcohol that is drinkable, when the Constitution prohibits it. The person who drinks this alcohol is a deliberate suicide." and "If the Senator's theory is that alcohol is so poisonous, then why put poison in it?" (the second quote is not a response to the first one).
The British Royal Navy served "grog" to its sailors. You could call it watered-down rum, but it's more like rummed-up water, and the main point was that the alcohol would keep the water drinkable. This was also done to moderate the sailors' drinking. The daily ration wasn't enough to get really soused anyway, but issuing it watered down discouraged sailors from hoarding up their rations and going on a bender.
Rationing in World War II led to this—in more ways than one. During the siege of Leningrad, the food situation got so precarious that the Soviets started issuing bread filled out with sawdust, and the Germans themselves would later give the foodstuff to POWs, impressed foreign workers and concentration camp internees.
More Truth In Television, and perhaps an inversion: A very minute portion of alcohol was always added to water in ancient societies to make it safe(r) to drink. (Even the Apostle Paul, otherwise rather famous for being an ascetic, recommends this in a letter to Timothy because the other had been having stomach problems.)
This was done with milk and even bread depressingly often. (Milk would be mixed with water, bread with chalk, plaster, grit and worse...) There's an incredible amount of legislation on this in older legal systems, often putting it on the same severity scale as theft and murder. Because, well, it kind of is murder to sell someone "food" with the nutritional value of cardboard.
- The Nestle boycott of the 1970's was partially related to the risks of watering down powdered infant formula.
Apparently, according to the Talmud, in ancient Israel and Babylonia, wine was made so strong that it was actually undrinkable unless mixed with water in a ratio of about 2 parts water to 1 part wine.
- In much of the ancient Mediterranean wine was fermented to a very high alcohol level and then watered down later, either soon before sale or by the customer. It was considered very unhealthy to regularly imbibe un-watered-down wine.
And as for good old fashioned watering down beer? The trick isn't to water it down with water. That gets spotted. Instead, you water down expensive beers with cheap beer.
Watering wine was a very important skill for bartenders in ancient Greece and Rome - you started strong and then began diluting as the customers got drunker and less able to notice. There's a famous bit of graffiti in Pompeii that assaults a caupon for watering his drinks too much.
- This is actually in The Bible even; when Jesus performed his first miracle, he didn't turn water into any old wine, he turned it into the very best wine, and one of the guests passes comment about it to the host, since the usual practice was to not just water it down but trot out a cheaper wine overall.
Averted with Janis Joplin. Apparently, she was just one of several people killed by a dealer who failed to cut his batch of heroin enough to make it "safe".
- This is a very common reason for a drug-related deaths actually. As established dealers try to capitalize on their status, they tend to progressively dilute their ware with various adulterants, forcing their clients to buy a larger and larger amounts of drug for the same kick. And when a new dealer moves in and tries to compete by selling a stronger, more pure stuff, the users very often use the dosage they got used to with the old, diluted drug for the new, more pure one, leading to overdose.
On a lighter note, diluting alcohol with sodas or fruit juices to make larger, less alcoholic beverages is one of the staples of modern bartending.