This is the point of every advert ever created for Herbal Essences shampoo. Most shampoo commercials knowingly hint at it, but these ones take it Up to Eleven with lashings of Double Entendre and scenes of women in the throes of very Immodest "Organic" experiences.
An ad for Head and Shoulders shampoo features an attractive young couple sharing a shower taking turns washing each others hair. The viewers is left with the impression that keeping your hair clean and dandruff will improve your sex life (which it probably will, actually).
When you apply Axe's Sex for Product marketing scheme to their line of body wash, you get this.
XXXenophile story "The Spirit is Willing" (sex with the ghost of a woman that died in a shower).
In the Jay and Silent Bob comic book mini-series, Jay tries to have sex with Tricia in the shower but she kicks him out.
This was actually implied in a Spider-Man comic. A caption that divided a scene with Peter and Mary Jane and a scene with Peter at ESU read "One breakfast or corn fritters and a shared shower later..." (This happened during Todd McFarlene's run as the artist, where the title tended to portray the couple as a little risque.)
Topper was a comic about a horny ghost that ran in the Australian edition of Penthouse. In one strip, he spied on two women in the shower of the change room at a tennis club, and gradually manipulated them into having sex.
In Apollo 13, this is where Jack Swigert is when he gets the call telling him he's become part of the flight's main crew.
Subverted in the movie White Sands (1992). Lane Bodine (Mary Elizabeth Mastrantonio) offers herself to (married) protagonist Sheriff Ray Dolezal (Willem Dafoe) while he's in the shower. The next scene has an FBI agent listening with amusement to the sounds of passionate sex over his hidden microphones...until Dolezal kicks down his door, whereupon we see Bodine faking the sounds as a distraction.
Averted in Life as a House. At one point, Jena Malone steps into the shower with Hayden Christenson (apparently with no intention of sex). He makes a pass at her, and they eventually start kissing. He then prematurely ejaculates, and she amusedly tells him that he'll be cleaning the shower.
Creator shower sex between Virginia Madsen and Vincent Spanno.
St. Elmo's Fire has Andrew McCarthy and Ally Sheedy breaking the shower screen.
Heartbreakingly rendered in Snow Falling On Cedars, when the young new widow, in the middle of a courtroom scene to determine the guilt of her husband's accused killer, flashes back to one of the last times she saw her husband alive: having Good People Have Good Sex in the shower with him.
Ana and her briefly-seen husband do this in the Dawn of the Dead (2004) remake. Below the neck you just see outlines.
In Daywatch, the protagonist chooses this way to reveal his feelings to his Love Interest (no sex, though, just heavy snogging). Bonus points for him residing in a woman's body in that moment. Hell, yeah!
In Pitch Perfect, when Beca goes to take a shower in the girl's locker room, two pairs of feet, belonging to a man and a woman, can be seen in a shower stall.
In Iron Man 3, Pepper Potts lures Tony Stark out of his workshop by announcing, "I'm going to take a shower. (Beat) You're gonna join me."
In the Molly Ringwald comedy For Keeps, her character showers with the boyfriend she married when they both move into an apartment together.
In the remake of The Getaway, the [McCoys] make their reconciliation official—she slept with an influential mobster in order to get him out of jail by having one of these that eventually moves to its proper location—a bed.
[Crusher] took a deep breath as if a weight had been lifted from her. "Okay, so... would you like to shower?"
"You can go first."
She smiled. "Who said anything about going one at a time?"
Live Action TV
Subverted in the premier of Alias, where Sydney lures her fiance into the shower with the implication she wants to have sex, when she really wants to use the noise the shower makes to hide that she's telling him about her real job as a spy.
In an early second season episode of In Plain Sight, Mary goes to her boyfriend's house and finds him in the shower. After they talk, he asks her for a towel. She opens all the cabinets and finds nothing but a washcloth. The next scene is the washcloth hanging on the door handle and her stepping into the shower. He'd replaced a suitcase full of drugs with the towels to foil the FBI, and no one had been able to explain the towels.
Charmed had one between Leo and Piper. A later Time Travel episode had Phoebe walking in on this scene.
Kind of subverted in Criminal Minds, when Garcia's talking to JJ about showering with Kevin. "Sometimes a girl's just got to wash her hair alone, you know?"
Kevin and Toby have at least one steamy shower sex scene on Dante's Cove. Memorable in part because it was used in the opening credits of season three.
Shower Scenes in general are fairly common in prison dramas like Oz, but a particularly notable Shower Of Love subversion occurs in season three: homosexual Richie Hanlon has just come off death row and returned to the general population. First night back, he's in the shower when another man starts coming onto him. Both Richie and the audience are expecting some celebratory "off-death-row" loving, until the other man slits Richie's throat with a razor blade he had hidden between his teeth.
The short-lived British sitcom The Savages had the main characters doing this, then wondering why it had been so long since they tried it. They remember it was because they could never agree on the water temperature.
Mentioned in the All in the Family episode "The Very Moving Day", when Gloria finds out that she's pregnant, and she and Mike blame each other for the time they had sex without protection:
Gloria: Well, there was a cold shower in the house, you know!
Briefly discussed in Cuckoo after Ken tell Cuckoo not to walk around naked. It's fine to be naked while having a shower, having sex with his daughter and having sex with his daughter while in the shower apparently.
Oddly given its general cheerfully explicit approach to sex and plentiful strips featuring the cast showering, Ménage à 3 mostly seems to avert this. Notably, Gary's shower with Amber is interrupted just as it's getting really interesting (strips #662-666, October 23-November 01, 2012, NSFW).
Deconstructed by College Humor in "Showering with Girls", in which Dan states that while he would like to see this trope happen, he goes into (perhaps too much) detail as to why it would be difficult, uncomfortable, or even downright dangerous in real life.
In The Simpsons episode "Kamp Krusty", Homer and Marge have a scene like this after Bart and Lisa leave for camp.