Night Swim Equals Death

" 'Tis a naughty night to swim in."

- William Shakespeare, King Lear

Many people enjoy swimming from time to time, it's fun and a complete sport. Maybe you even like taking the waves. Or maybe you just like the relaxing feeling of the sea. Or maybe you enjoy the water in company. But if you are in a fiction work you should only go for a swim in broad daylight, otherwise things tend to get a little messy...

No matter the place: the beach, a lake, a river, open sea or a swimming pool, if you go for a swim after dark you will die in the water or shortly after. It may be a Sea Monster out for dinner, it may be a Serial Killer stalking you, it may be an Eldritch Abomination coming out of the sewers or maybe you manage to drown by yourself but no matter what, you are in for a gruesome death.

This can be Truth in Television to some degree, as swimmers are less likely to see aquatic predators such as sharks and crocodiles at night, and such predators are often more active at night too. This is why many beaches close at night or at least warn people that night swimming is dangerous. In a similar vein, a human killer can take advantage of the water, darkness and often solitude of a night swimmer.

Compare Darkness Equals Death where darkness means trouble, Hazardous Water where the water indirectly contributes to your death and Murder Water where the water itself is trying to kill you.

Film

 * Frequent in the Jaws movies. The opening scene in the first movie is the most remarkable one.
 * Happens to anyone who skinnydips at night in Crystal Lake in the Friday the 13 th movies.
 * Parodied in 1941 - girl goes swimming at night, gets caught on a Japanese submarine. It's even the same girl who did the Jaws scene.
 * Subverted in Cat People. Although it doesn't end with a death, the suspension being built up pretty much forces you to expect it
 * Averted in classical Finnish films ("Suomi-filmi"); skinny dips, nighttime or not, bring no greater harm than perhaps an unwanted admirer.
 * Deep Blue Sea plays with this trope. It's subverted in the beginning as it seems everyone in the water is going to die, but they are saved by the protagonist. However later it's played straight and often, as most of the movie takes place during night.
 * Averted in Gattaca, when Vincent and Anton go for a swim in a lake/ocean (it's not explicitly stated and there's minor problems with either option) at night during the film's climax as a Call Back to when they did the same towards the beginning. Vincent is stated as being at risk for a heart condition, and Anton was shown to be a better swimmer in the beginning of the film. Anton becomes afraid and gives up, while Vincent has already resolved that he will succeed or he will die, and only stops to go back and help his brother.
 * Parodied in the opening scene of Piranha, in which a couple break into the research facility to take a dip in the pool and get eaten by the piranha inside.

Literature

 * In the Backstory to The Reincarnation of Peter Proud, the title character goes for a swim at night. His wife rows out to him in a small boat and knocks him unconscious with a paddle, causing him to drown. The same thing happens in The Film of the Book.
 * One of the murder victims in A Caribbean Mystery falls foul of the killer while swimming at night.
 * In the book Siren the older sister drowns when she decides to go and dive off a cliff for fun in the middle of the night

Live Action TV

 * Too many CSI, CSI: Miami and CSI New York episodes to list. When this happens the usual is finding the body floating in the swimming pool.
 * In Primeval, a couple is swimming in a public pool at night, when a mosasaur joins them...
 * In an episode of Brimstone, a young woman (who is one of the 113 escapees from Hell who makes for the show's McGuffin, and who went to Hell after she murdered the families of the men who raped her) goes swimming at night with her new boyfriend. When he gets a little too frisky and triggers her, her demonic powers kick in and she boils him alive in the water.
 * Happens in Buffy the Vampire Slayer in the episode where the swim team are turning into fish monsters.
 * In Smallville Lana Lang once went for a night swim, at the high school pool if memory serves. Naturally she was attacked, and Clark saved her.

Web Comics

 * The plot of Schlock Mercenary's Mahuitalotu arc kicks off this way.

Western Animation
"Lenny: Oh no!, sharks! The assassins of the sea! ...wait, you're Dolphins, the clowns of the sea.
 * A Halloween episode of The Simpsons has Lenny recreating the opening scene from Jaws, except with dolphins in place of a shark.
 * The dolphins proceeded to beat and bite him to death*."