In the real world, swimming is a skill that usually must be learned over a period of time - and that's just for swimming on the surface. Swimming underwater is another skill all on its own that must be learned, and even then, no human can stay under for more than a couple of minutes or so at the most without the aid of some special equipment, or some continuous training to boost lung capacity.
In fiction, however, this appears not to be the case. Video games are the biggest offender here. Any character that doesn't have Super Drowning Skills has a completely natural ability to swim like a fish, even if they've lived out in someplace like a desert their entire life.
Usually, though, they must acquire some special outfit or equipment in order to be able to swim underwater - but once they do acquire it, they are immediately able to use it like a total pro, and don't require any practice using it or moving around in the water with it whatsoever.
Then there's also the fact that in many video games, characters can swim and tread water for indefinite amounts of time without getting tired. In Real Life, swimming exerts a great amount of physical strength and energy, and gets very, very exhausting after some time, even for Olympic Swimmers (the namesake of this trope). Of course, they're also capable of walking for hundreds of miles on dry land without a break either...
This may be a somewhat Acceptable Break from Reality, however. If characters in video games had even remotely realistic swimming skills, it would take a lot of fun out of exploring underwater ruins and such, and not to mention it would be extremely aggravating for players.
- In Paperinik New Adventures, Korinna swims at an incredible pace and seems to be able to hold her breath for incredible amounts of time.
- Subverted with The Poseidon Adventure. Belle used to be a champion underwater swimmer, but she's years older now, and although she makes it through to the other side of the underwater area and saves the reverend, it's too much for her heart and she dies.
- Mermaids has Kate Flax who actually wants to be an Olympic swimmer. She can already swim astoundingly well for a kid of her age unless she's drunk.
- In Percy Jackson and the Olympians, this is one of the powers that Percy has inherited from his father Poseidon, god of the sea.
- Poseidon is a Greek god, sometimes known as an Olympian. This makes his son Percy a literal Olympic Swimmer (in that he is descended from an Olympian, not actually competing in The Olympics).
- Prince Caspian discusses and averts this trope when the children are contemplating swimming a river to escape from the island they're trapped on. They note that the only one of them who would stand a real chance of actually making it to the other side is Susan, who has "won prizes for swimming", but Susan herself points out that without knowing the river or its currents, swimming it would be a ridiculously dangerous thing to do.
- The Hunger Games: How Annie Cresta won her games, outswimming the other tributes. Justified, as she lived in the district responsible for fishing and would have grown up being taught relevant trade skills like swimming.
- In the Assassin's Creed series, starting with the second game, the player character always has perfect swimming skills: never gets tired and never sinks no matter how much armor he or she is wearing. In Assassin's Creed III, the player can also swim in icy-cold water without suffering hypothermia or frostbite. There is a concession to realism in that you can only hold your breath underwater for a certain amount of time (in gameplay this grants total concealment from enemies) before having to come up for air. All other characters in the series have Super Drowning Skills.
- In the Command & Conquer: Red Alert Series, Tanya Adams can swim across any sized body of water without slowing down to sink a dreadnought with C4. A few other infantry units from all factions in 3 are this as well
- Final Fantasy Crystal Chronicles: Echoes of Time: In contrast to the previous game, the hero and whatever allies s/he picks up can swim indefinitely regardless of whatever heavy armor and weaponry they have equipped (one dungeon is entirely swum). They can also dive, and though they swim back up immediately, the player can just keep hitting the dive button over and over to keep them underwater with no ill effects.
- Grand Theft Auto: San Andreas allows the player to swim indefinitely and never grow tired the first time they ever jump in the water.
- Half-Minute Hero: If Hero doesn't have a boat, he'll just swim across the ocean.
- Both Everquest and Everquest II have a breath mechanism, but beyond that swimming is just another movement mechanic. Both games have a Swimming skill but its only effect on the game is how fast you move when swimming. You can swim in any water forever without tiring or freezing, you can tread water forever and if you have any item or spell that eliminates the breath meter (underwater breathing or a Fishbone Earring or whatever) you can swim forever if you care to do it. In Everquest II you can swim from one continent to another if you have the time (zone edge mechanics block this behavior in Everquest).
- Lampshade hung in World of Warcraft with Griftah the charm salesman, who sells charms that grant amazing abilities...that you already have. One of them (supposedly) gives your character amazing swimming ability.
- Super Mario Bros.: The titular Mario and Luigi. Apart from their Super Not-Drowning Skills, they've actually managed to out-swim torpedoes and can dash through the water in Super Mario Galaxy by spinning, using their arms as propeller blades. Lampshaded in The Adventures of Super Mario Bros. 3, where Luigi points out that Mario can swim better than most fish. Sometimes other characters are this, as well, but not very often, and certainly never quite as swift as the Bros.
- In Dwarf Fortress, your dwarves (and most other non-aquatic creatures) require at least novice skill in swimming to avoid drowning, and at least adequate skill to avoid drowning when stunned, which usually occurs when falling into the water. Other than that the swimming skill only affects speed, and swimmers only tire when selecting a swimming "gait" above the medium speed, in the same way they only tire when running or sprinting on land.
- Zig-zagged in the various games of The Legend of Zelda series.
- Probably justified in A Link to the Past. Link starts out with Super Drowning Skills, but is then able to swim indefinitely after he gets an item called the Zora Flippers, which are magic, presumably explaining why they allow to swim forever.
- Played straight in Ocarina of Time, Majora's Mask, Twilight Princess and Skyward Sword, where Link can swim for an indefinite amount of time on the surface of the water. Underwater swimming does have a time limit until you get the proper items, but that's a different trope.
- Averted in The Wind Waker and Breath of the Wild where there's no underwater swimming and surface swimming is limited by a stamina meter. Because the former game is set on an ocean, this was done to prevent the player from traveling between the islands without the boat. The latter game zig zags when it comes to the Zora Armor; it only slightly increases your swimming speed, but it also gives you the ability to swim straight up waterfalls rapidly. It's also somewhat ironic, since the Link from Breath of the Wild grew up around a lot of Zoras, and yet is a far worse swimmer than the Links from the above mentioned games, whose only exposure to large bodies of water would have been small rivers and ponds.
- Present throughout The Elder Scrolls series. The Player Character, regardless of race of background, can swim perfectly well even if they are in a full suit of armor and carrying hundreds of pounds of gear. Even the Khajiit, a Cat Folk race with a desert homeland, can swim without issue. Arguably Justified for the Argonians, Lizard Folk with gills who hail from a marshy homeland, who are canonically good swimmers (even conducting underwater guerilla warfare with this Exploited Immunity). Less justified is that they can even do this in, say, the frigid waters of Skyrim. This also applies to NPCs who, whether they are following you, chasing you, or simply accidentally wind up in water, can swim without issue.
- Subnautica: For someone whose role was essentially head janitor, the protagonist can certainly handle himself like a pro underwater. While lung capacity isn't that hot without proper breathing tanks, he never, ever gets tired even if he has to swim literal miles.
- Animalympics features actual Olympic swimming, but Animalympics being Animalympics, it takes it beyond Olympic swimming: When caught by a massive wave caused by his opponent, the ginormous Japanese orca Ono Nono, the Californian otter Dean Wilson resorts to riding through the tube, using his own tail as a surf board. Granted, being from So-Cal, he is trained in doing this well enough to keep cool all the time, but he has certainly never trained switching to surfing while in the middle of a swimming race.