Breast Sponge

Given half a chance, an Anime featuring an Unwanted Harem will put the male protagonist and a female love interest in bath together. Which is all well and good, Fan Service-wise. Of course, sometimes they want to add some extra Ship Tease to the scene. As You Know, it is very difficult to scrub one's own back. So the girl will offer to help the guy out by washing his for him.

This has the bonus effect of turning the fanservice Up to Eleven. The guy's reaction can range from horrified pleasure to indulgence. There's a very good chance she'll point out that this allows her to clean her front and his back simultaneously. Don't be surprised if he gets a Nosebleed. In even racier media, she may ask that they reverse the positions. Not What It Looks Like is very likely.

Anime and Manga

 * In Midori no Hibi, this is performed with an absolute minimum of sexuality (Midori keeps herself wrapped in a towel). The "clean myself at the same time" line gets used nonetheless.
 * Kodomo no Jikan: Rin offers to give her teacher a back wash, but realizes she can't do it properly because
 * In Seto no Hanayome, while Nagasumi is under the effect of a potion that makes him super attractive to women, nearly the entire female cast follows him into the bathroom for a Furo Scene, aiming for this.
 * In the third episode of Asobi ni Iku yo!, this makes an appearance, along with any number of other Furo Scene Turn-On Tropes.
 * Urara fropm Sakura Diaries is infamous for offering and giving these to her love interest, her cousin Touma.

Comic Books

 * The Caged Demonwolf imagines Empowered doing this to a weakened and traumatized Ninjette. (Yes, Emp gives her a bath, but the truth is less exciting than that.)

Video Games

 * The Eroge The Sagara Family has a fully sexual scene of this type.