Walking Swimsuit Scene

For some reason, some girls seem to walk around in a swimsuit only for Fan Service, which may be justified because they work at a swimming pool or at the beach.

These girls don't have to wear only a swimsuit; they can still wear other clothing as long as we see that there is a swimsuit underneath. For example, Misty of Pokémon Gold and Silver wears a jacket and sandals, but we can see that she's wearing a swimsuit underneath.

Contrast Leotard of Power, Walking Shirtless Scene (though it can cross for guys), Underwear of Power.

Anime & Manga

 * Yukimura Sanada from Hyakka Ryouran Samurai Girls wears a White School Swimsuit under her clothes. It's MUCH more apparent on her Master Samurai form.
 * One Piece
 * Franky is a male example. Being a cyborg, it doesn't matter if he's Exposed to the Elements.
 * Kiwi of the Square Sisters and some of the Kuja, like Sandersonia and Daisy, are straight examples.
 * Kenko Zenrakei Suieibu Umisho revolves around the wacky antics of a swimming club, so the cast is wearing School Swimsuits most of the time. Amuro even wears a swimsuit under her school uniform so she can take a swim at any time -- when she's wearing anything at all, that is.
 * Tengen Toppa Gurren Lagann has Yoko.
 * Sayaka Yumi spends her day in Mazinkaiser's 4th OVA in a swimsuit not only on the beach, but an enemy's base because she's, well, kidnapped.
 * Edolas Bisca from Fairy Tail.
 * Cana is usually in a bikini with pants and an occasional jacket.
 * Lucy and Erza too, during the Tenrou Island arc
 * Strike Witches has Yoshika, Mio, and Junko, all of whom wear School Swimsuits underneath their uniform tops. And no pants.
 * Medaka Box has Kikaijima Mogana, which makes sense because she's on the swim team. However, it's more of a security issue than it is one of practicality.
 * As a member of the Swimming Club, Shirley from Code Geass gets a few of these in her School Swimsuit.

Comic Books

 * Tigra of The Avengers is a Catgirl (though of the "covered in fur" type, not the "human with cat ears and tail" type) superheroine whose "costume" is a black bikini, sometimes with toothlike trim or a tiger-head amulet as a clasp in front.
 * Sub-Mariner
 * Namor wears a black bodysuit on the rare occasion when he feels an ounce of humility, but he's mostly known for going around in nothing but a green speedo. Somewhat justified by the fact that he spends most of his time underwater, but mitigated by the fact that (1) he wears it on land too, even in frigid climates -- in fact, until the 90s or so, it was the only thing he ever wore, and (2) the vast majority of his fellow Atlanteans cover up quite a bit more.
 * Namor's two distaff counterparts, Namora and Namorita, fight crime in a black one-piece and a green one-piece or bikini, respectively. When Namorita changed her name to Kymera (and her skin to blue) she adopted an odd bikini/netting/seashell ensemble.

Films -- Live-Action

 * Many Fan Service Extras in the James Bond films. (Commonly included in any list of "Bond Girls", but Bond Girl refers to characters with actual personalities and prominent role in the movie)
 * In Local Hero, Marina, a marine biologist, wears a black one-piece under her lab coat at a facility -- justified as she has to do some equipment maintenance at the bottom of a large water tank. The visitors are quite distracted.
 * The Bikini Car Wash Company
 * The female characters in Into the Blue.

Live-Action TV

 * Every female character on Baywatch, ever. Justified for being Lifeguard and all.
 * Miss America pageants.
 * Every lead female character on Saved by the Bell ends up doing one of these at least once.
 * Heather Thomas in the first episode of The Fall Guy - and then repeated in the opening credits of every single episode thereafter, for obvious reasons.
 * This was something of a standard trope in the 1970s and 1980s; if the cast of a series (sitcom or detective/action mostly) had a sexy female member, odds are are least one episode would find an excuse to get them into a swimsuit, either for a contrived beauty pageant plotline (i.e. The Bionic Woman) or by finding an excuse to get them next to a swimming pool or on a cruise ship (i.e. Wilma Deering in the cruise ship episode of Buck Rogers in The 25th Century).
 * OK, it's not a swimsuit in the traditional sense, but Wonder Woman has to count.
 * And Wonder Woman's swimsuit showed much less flesh than her normal costume.
 * Male version on Buffy the Vampire Slayer as the girls discover Xander has joined the swim team. When he sees them ogling him he goes into a horrified crouch and covers himself with a paddleboard.

Video Games

 * Misty from Pokémon Red, Blue, Gold, and Silver and their remakes. An odd case, since she just wore swimsuits prior to the Time Skip. Possibly because an eleven-year-old walking around in a swimsuit is more innocent than a teenager doing so.
 * Arcana Heart
 * Kira Daidouji, because she fights waist-deep in a blob of water.
 * Her rival, Catherine Kyohbashi for no reason whatsoever.
 * Rikku from Final Fantasy X-2 wears a bikini with a miniskirt and a scarf over it as her default outfit.
 * A Non-Fan Service one, In My Sims Agents, Clara Belle, Taylor, Summer and Luke wear some Beach-Wear (though Summer is an Lifeguard).
 * Dead or Alive: Xtreme Beach Volleyball
 * Justified with Trauare Wrede in Rosenkreuzstilette. Due to her status as the game's Water-element boss, she wears an open jacket over her swimsuit, and, well... not much else. It's as hot as it sounds.