Reed Snorkel

""I don't care if you did do it in a movie, Gilligan is not breathing through that reed!""

- Gilligan's Island

Plucking a hollow reed and using it to breathe while underwater.

This is an example of Reality Is Unrealistic: the major problem is not the pressure, which only becomes a problem as you go really deep (most people are swimming just a few inches below the water, hardly low enough to crush your lungs), but that the width of the reed (or snorkel) needs to get wider the longer the tube is. Otherwise you are just rebreathing the same air over and over, which will kill you after long enough. Unless you breathe out into the water, which defeats the purpose by highlighting your position with bubbles. Hmm... maybe you could do it with two reeds.

Anime and Manga

 * Orochimaru does it to listen to Team 7 in the Forest of Death in Naruto; significantly, he isn't underwater.
 * Done in Mahoraba during a game of hide-and-seek.

Comic Books

 * Jimmy Olsen used this to hide from an assassin during the lead-up to the New Krypton storyline, with the added complication that the assassin could read minds. The only explanation Jimmy could think of was that all the times his body had been transformed over the years had made his brain impossible to read.

Film

 * Robin Hood, in...well, Robin Hood. The anthro one.
 * James Bond in Dr. No.
 * The Three Musketeers 1973 had d'Artagnan try this in a horse trough. Rochefort gets frustrated that he's 'lost' d'Artagnan and kicks the tap off the trough...causing it to empty and leave d'Artagnan exposed.
 * This is used to hide from guards in I Am a Fugitive from a Chain Gang.

Literature

 * The Phantom of the Opera (novel version) not only did this, but he would also sing through it to get people to look overboard so he could pull them under.
 * Referenced in one of the Tiffany Aching Discworld books. Miss Tick considers being able to breathe underwater through a hollow reed after being tied up and thrown into a pond an essential survival skill for witches traveling through places where they aren't welcome.
 * The hero of the first book of the Sienkiewicz Trilogy uses this trick to escape from a besieged city.
 * Various Redwall characters use Reed Snorkels.

Live Action TV

 * Rated "Plausible" by the MythBusters. They could breathe while remaining concealed underwater using a reed snorkel, and with a little practice they could also make it double as a blowgun, and hit a target above the water.

Video Games

 * In Tenchu 2, you can use this to become invisible in the water, which is slightly jarring as the water is depicted as crystal clear.

Web Comics
"Elf: How do you know these reeds are hollow? Kevyn: Reeds are always hollow. (Either Kevyn is referring to some kind of botanical rule - which seems unlikely, since he's a professor of subspace physics and not xenobiology - or he's directly referencing this trope.)"
 * Used in Schlock Mercenary. On an unknown, alien planet.

Western Animation

 * Race Bannon does this while approaching the tribal camp in the Jonny Quest TOS episode "Pursuit of the Po-Ho".
 * Sylvester tries this with a lead pipe to "save" Tweety, who is stranded by high tide, in one Looney Tunes short. Hilarity Ensues when a seagull decides to roost on the pipe.

Real Life

 * "Brady's Leap". Sam Brady hid underwater breathing through a reed stem while escaping from Indians.