I saw a IMAX film about the threatened life of the reefs of the Pacific. They showed various fish which share a symbiotic relationship, such as some shrimp-like critter that feeds on the good stuck to the teeth of a larger fish. Then, the diver took off her mouthpiece (from the air tank) and tried it herself. But, like, why didn’t she drown??? Shouldn’t have lungs have instantly filled with water?
Oops! I meant the gook stuck to the teeth! :smack:
Of course you can open your mouth underwater. Haven’t you ever tried it? I suppose the back of your tongue just naturally blocks off your throat. I’ve never really given it much thought.
You will be shocked to learn that on occasion, I pour liquid directly into my open mouth and it rarely goes right into my lungs. 
The epiglottis blocks the windpipe. That’s why you don’t die from drinking water.
In that circumstance, your epiglotis is at work. When not eating/drinking, it’s in the wrong position. Or, did you learn to swim with your mouth always open?
Have you never been swimming? It’s perfectly possible to open your mouth under water. You just don’t breathe in at the same time.
A lot of people seem to not be at home in the water. Maybe it’s the whole not breathing thing or something.
You can open your eyes underwater. You can open your mouth underwater. You can even drink and eat underwater.
Wiseguy yourself; it is quite possibly to swim with your mouth always open. Do you own a bathtub? You can try this really complex experiment by yourself!
<b>Silentgoldfish</b>, you seem pretty smart. How come, when I drink a glass of water, it doesn’t immediately pour out of my ass? I mean, like, there’s a hole down there, so what keeps the water in? I just don’t get it.
I’m simply astonished that someone could go their entire lives and never tried this. Do you live in the desert and only take sponge baths? I’m not trying to be rude here but it’s kind of amazing that anyone wouldn’t know this.
I can open my big mouth just about anywhere.
Underwater so far has proven to be the safest place for me.
What if he accidently Bapistizes himself?
In all seriousness, I’m assuming that at some depth the water pressure from outside would be greater than the air pressure from within the lungs, which would cause water to go into the lungs, no? Or is the epiglottis strong enough to keep water out up to or past that depth (or perhaps the body itself couldn’t withstand that pressure and you’d be crushed anyway)?
No, because your body is soft and squishy (bones, notwithstanding). As a result, the pressure inside your lungs is equal to the pressure outside, provided you don’t try to breath in, of course. This is, in fact, the major reason your lungs don’t fill with water when you open your mouth, not the presence of the epiglottis.
You are mostly water yourself. As you go deeper, the pressure squeezes you down some ( your ribs give a lot more than you might think ) and your lungs are not a hollow drum, they are more of a sponge like deal. There is not that much empty space in your body.
Free divers can get to 300+ feet ( record is about 400+ IIRC )and can open their mouth just fine. There is prolly a depth that the pressure would over come your ability to lock up your throat but it would be so deep that you would have already died from other problems. ( you can usually equalize the pressure on your ears too. ( Don’t try this without practicing… )
YMMV
As an aside, a crazed belgian just established a new deepest-ever breathold dive mark by using a technique where he flooded his sinuses and other head cavities with seawater in order to save time on equalization and conserve air. Ouch!
And yes, one of the standard PADI learning exercises is having to take off all your gear underwater (including taking the mouthpiece out) and then putting it all on again. So I have done the whole ‘open wide underwater’ routine and nothing serious happens - as has been pointed out by many others, your throat/tongue/epiglottis is pretty well adapted to keeping stuff in the mouth from going down the windpipe/oesophagus without your say-so.
Kids and public swimming pools – stuff coming out of both ends.
Check out answers.com for a very simple explanation.
For a more in depth (no pun intended) there’s deeperblue part one and part two
And last, there’s the more esoteric study
Does anyone remember the TV series Man From Atlantis (starring Patrick Duffy)? there were numerous open-mouth-underwater scenes in this, as indeed there have been in many movies and quite a few pop songs (the one that springs immediately to mind is All In My Head by Kosheen*).
And as anecdotal testimony, I can open my mouth underwater quite comfortably; if you allow your mouth to fill with water (without swallowing it down), you can even speak short words and phrases underwater - longer ones cause you to expel the water and produce bubbles, breaking up the sound and spoiling the effect.