Forgot I Couldn't Swim

"Terry instinctively flicks his wrist, as if trying to produce a Batarang, then remembers he's not in costume--dope!"

- Original script for Batman Beyond: Return of the Joker

"Oh, this vehicle was never designed to run underwater. That was important and I forgot it!"

- Freakazoid! after running his car into water

There are many situations where applying the skill of swimming is very useful. Say, for the sake of winning a bet. Recovering lost or damaged property. Saving someone whose life you care about. But there is one situation where no person in their right mind should ever try swimming, for any reason.

That is when the character in question does not actually have the physical ability to swim.

This trope is usually used as a gag. Usually a prideful or heroic character will quickly make a snap decision to swim without thinking through all the ramifications first. They're just that focused on the immediate goal. We as the audience can laugh at them for their own foolishness in being so forward without bothering to plan correctly.

Conversely, this trope can be played for major tragedy if as a result of all this someone is actually seriously physically hurt. Even worse if the only person around was the one who made the leap, and thus also incapacitated themselves and made even more trouble for the eventual rescuers.

Note that, name notwithstanding, the activity in question is not necessarily swimming, much like Forgot I Could Fly. If they actually do swim very well in the attempt, the reminder that they can't is invoking the Centipede's Dilemma.

A Sub-Trope of Remembered Too Late.

Anime and Manga

 * In Ranma ½, Akane first establishes her Super Drowning Skills with this gag, jumping into the ocean to save Ranma from Cologne's attack failing to remember that she is utterly helpless underwater.
 * In a very similar joke, Ryouga once dives into the water to save a drowning Akane, completely forgetting that he'll turn into a small black pig in contact with the water. Ranma even lampshades this mid-dive.
 * During the Pantyhose Taro arc, this antagonist kidnaps her and takes her to a remote temple in the mountains, then leaves to confront Happosai. Akane escapes, comes across a river, and leaps in... only to splash and sputter her way back to shore. She even says "Oh no! I forgot I can't swim!" afterwards.
 * In School Rumble, Tenma discovers that while she was absent, her three friends made a bet with some guys that they could win in a swimming race- the prize being dinner at Eri's favorite (expensive, high class) restaurant. When the race comes.
 * Not quite this trope;
 * Shows up in One Piece. This is a reflex which most Devil Fruit users should get out of their system early, seeing as they're pirates and all.
 * ...Let's explain that real quick. Anybody who eats a "Devil Fruit" gains some kind of fantastic power, such as an elastic body, at the cost of their natural buoyancy. Should they fall in the water, they will sink like an anchor and become paralyzed as well. Since almost the entire cast of One Piece are pirates and live on the ocean, this is a pretty big deal.
 * And it's a hilarious running gag where Luffy, the elastic main character who ate a Devil Fruit as a kid, falls into the ocean and begins to drown. Chopper and Brook valiantly leap in to save him... but they're Devil Fruit users too. All three of them are then saved by one of the other crew members who're understandably pissed at their thoughtlessness.
 * At least Nico Robin, the final Devil Fruit user in the crew is smart enough not to ever do this. She only has one recorded instance of falling into water and in that case the risk beat the alternative.
 * In Digimon Adventure Jou (Joe) saves Takeru (TK) from drowning before going under himself. Thankfully, his actions cause his Crest of Faith (Reliability) to glow, evolving Ikkakumon into Zudomon and saving both of them.
 * Early in Kodomo no Omocha, Sana challenges Akito to a bungee contest, since she knows that he's afraid of heights. Meanwhile, she forgets that although she loves high places, she has no idea how she'll react to falling from them—as it turns out, with abject screaming horror. This is in character.
 * Nobita from Doraemon is a good example to this. While saving a friend, a character usually states that he couldn't swim. Realizing that, he drowns, usually ending up being saved by another character.
 * In Yoake Mae Yori Ruriiro na, Feena jumps into the river to save a drowning puppy despite the fact that she herself can't swim. Luckily, Tatsuya is there to rescue her.
 * In Azumanga Daioh, in the first volume, Chiyo can "sort of do a doggy-paddle," but that doesn't stop her from jumping in and proceeding to sink. Of course, this happened during swim class and had many people monitoring her, so teaching her was kind of the point. (She improves as the series progresses; by the time they go to Okinawa, she's been SCUBA diving.)
 * In Dino Zaurs, Rena jumps into a hippo and crocodile infested lake in Africa to try to goad her friend into jumping in as well. He jumps in, but only to save her when she nearly drowns.
 * Happens to Nagisa from Animal Detectives Kiruminzoo in the second episode.
 * In the anime version of Asu no Yoichi, the title character was raised in the mountains and never learned to swim, but when a side character falls into the water and is unable to swim because the bad guys are creating waves preventing him from doing so, Yoichi heroically jumps in to save the other. After several moments, he surfaces and splashes about. The original water-bound character is eventually saved by a third character who can swim (though no one knew that) while Yoichi gets a life preserver tossed to him so he can distract/fight off the opponents.
 * In one episode of Sgt Frog, Keroro falls into icy water and starts to drown. In a display of friendship, Fuyuki forfeits a match to jump in and save him. However, previous episodes have made it plain that Fuyuki can't swim, and when Keroro points this out, he starts to flounder, even shouting out the trope's name verbatim.
 * In Fushigi Yuugi, Tasuki gets thrown overboard when the party's ship is in the middle of a storm. Knowing that Tasuki couldn't swim, Miaka heroically dives in after him... only to be reminded by Tamahome that she herself couldn't swim any better.
 * Again, in the second OVA series, Tasuki is reminded that he couldn't swim after Chichiri reminds him, after he revives Miaka from almost drowning.

Comic Books

 * In the Marvel Universe, the Taskmaster can copy any ability he sees perfectly. When he was little, he used this to perfectly mimic a dive when he was unable to swim and almost drowned as a result.
 * In a Star Wars one-shot comic, Boba Fett is hired to track down the man who killed a crime lord's son and kill him. Turns out, Boba Fett was the one who killed the boss's son, and the job was a setup to allow his other son to get revenge. After fighting against hordes of scrapyard droids, Fett douses the guy in oil and lights him on fire. Naturally, he runs screaming into a nearby pool to put himself out—remembering immediately afterwards that he can't swim. As he watches him drown, Fett aptly remarks "No smarter than your brother."

Film
"Game Master: Aren't you forgetting something? Ambrose: What? Game Master: Your character's crippling fear of water? Ambrose: (out of character) Oh, right. (in character) Aaaaaaaaaaaaaaah!"
 * Subverted in Time Bandits, when, after declaring as he struggles in the Atlantic after the Titanic sinks that he can't swim, Fidgit the Bandit discovers to his sudden delight when transported to the Time Of Legends that, for no adequately-explained reason, "Hey! I can swim!"
 * In The Gamers:

Literature

 * In Anne of Green Gables, Anne starts a writer's club. She writes a story in which two women fall in love with the same man, and the man falls in love with one of them. The other woman gets jealous, and eventually pushes her friend off a bridge into a river. The man they were fighting over jumps in to rescue her, but he can't swim, and both of them drown.
 * In Aliens Stole My Body by Bruce Coville, the main character has had his brain transferred into the body of an alien. He talks the alien whose body he is sharing into going for a swim, only to realize that's not a good idea when you breathe through your skin...

Live-Action TV

 * In one Degrassi Junior High plot, one boy makes a bet with the girl swim team that the soccer team can beat them at swimming. He does this in spite of the fact that the rest of the soccer team clearly has a look on their faces that this is a very, very foolish idea.

Music
"Cupid really made me stupid (Stupid so it must be him) I forgot I couldn't swim So I terrified my baby (Guess it wasn't very cool) Yes I made myself a fool (Lying to my baby) Someone should explain her maybe (Now the story is getting cruel) Why I drowned in the pool"
 * The song Swimming in the Pool by the Belgian group The Radios ends:

Video Games

 * In Ghost Trick, the protagonist Sissel is told to break into prison and read a certain prisoner's work schedule. Easy task for a ghost. But when the time comes, Sissel remembers he can't read.

Web Comics

 * In Schlock Mercenary, one arc has the titular mercenaries take a job as mall cops. As part of the contract, they have to leave behind all their military grade gear, including armor that can let them fly. When what appeared to be a thief started jumping from balcony to balcony, Captain Tagon jumped from his balcony intending to fly and follow. Only on his way down did he remember that he couldn't fly because he wasn't wearing his suit and promptly crashed into someone's dinner table, taking a fork in the eye for his trouble.

Western Animation
"Freakazoid: "That was important and I forgot it!""
 * In an old Fantastic Four cartoon, when the Fantasticar is shot over the ocean, the Four manage to bail onto an island. Johnny Storm turns into the Human Torch to go recover the submerged car. Think that one through, since clearly he didn't. Bizarrely, this is a dramatic variant of the trope's use, even though there's no real tension as to whether Johnny will be rescued. Eh, what're ya gonna do?
 * One of the many ridiculous plans to catch the pigeon in Dastardly and Muttley in Their Flying Machines was flying a diving board plane with Dick Dastardly on it, so when he was directly above the pigeon, he would dive, catch the pigeon and land on a plane carrying a huge tank of water. Before the squadron takes off, Dastardly remarks, "OK...but I still think we're forgetting something." Remarkably, Dastardly actually catches the pigeon, but suddenly says, "Uh oh! Now I remember! I CAN'T SWIM!", sinks beneath the water, lets go of the pigeon and once again begs for Muttley to rescue him.
 * In an episode where Dick Dastardly went on vacation, he dove at a pool but, because of another attempt from his squad to catch the pigeon, we never got to learn if the trope would repeat or if he had already learned by then.
 * Variation: Freakazoid! is using his super-car to distract Longhorn, a redneck minotaur in a super semi-truck. They race all around the country till they get to the ocean, and Freakazoid thinks, "Let's see him follow me under water!" Longhorn's truck has a scuba mode. The Freakmobile doesn't.

""I'll just swim around and drink as much as I want! Ha! (beat) Help! I can't swim!""
 * Happens a few times to Bullwinkle in Rocky and Bullwinkle.
 * Fry and the Slurm Factory:

""Oh no, I just remembered, I can't swim!""
 * In an episode of Danger Mouse, Count Duckula dives 500 feet into a glass of water. Guess what he says just after he makes the jump?


 * Phineas and Ferb: after Perry the Platypus switches bodies with Candace he once still tried to give a slow-motion tail-slap to Dr. Doofenschmirtz, but just span around pointlessly because he didn't have a tail anymore. After getting mocked for this, he then starts beating Doofenschmirtz with a purse from nowhere.
 * In The Zeta Project, the titular Do-Anything Robot has a wide variety of tools. A means of flight is not among them, which would have been nice to remember before he jumped out of a skyscraper.
 * In one Pepe Le Pew cartoon, Pepe sees Penelope on a cruise boat and runs after it. As he sinks he calmly tells the audience, "Silly me, I forgot I cannot swim."
 * In one episode of Codename: Kids Next Door, Numbuh 4 dove at an aspargus sea to chase what he believed to be a cheeseburger and then he remembered he couldn't swim.
 * In The Owl House

Real Life

 * Real Life Darwin Awards example: This guy.
 * It's funny when you're on a boat and you pretend like you're about to fall in. It's less funny when you can't actually swim, but do actually fall in. It's downright tragic when two other people who can't swim jump in to try to save you.
 * Richard Pryor mentions this in one of his routines. Whilst playing around with his children in his swimming pool, he accidentally jumped into the deep end and only after he was in the water did he remember that he can't swim.
 * The highly endangered Kakapo is known as "the bird that forgot that it forgot how to fly". Despite being completely flightless, they will throw themselves out of trees to try and escape danger.