In any case, the diver could have blocked off the entrance to her throat with her tongue, by pressing it to the roof of her mouth. This would have allowed access to her teeth without risking water forcing its way past the glottis.
Nice, but it has nothing to do with the specific question at hand.
For some reason my wife can’t do this thing where we ordinary people just sort of shut off the backs of our mouths. She even has to hold her nose shut with her fingers, if she goes underwater at all.
Any ideas why? Which bit is she actually missing? I’m going crazy trying to tell her which muscle to flex, because I’m not sure how I do it. She seems to think she simply doesn’t have a particular flap of skin, or muscle, or something.
She might have a cleft palate or shortened soft palate. Has she ever been examined by an ENT (ear, nose, and throat) specialist?
[QUOTE=Jinx]
Then, the diver took off her mouthpiece (from the air tank) and tried it herself.
[QUOTE]
Wait a minute. What exactly did the diver try to do? (Besides opening her mouth underwater)
I’m reading this as she tried to eat the gunk off the fish. And that just seems crazy.
You don’t need to block anything to let water into your mouth without inhaling/swallowing it. I don’t know if the throat closes up or the pressure in the lungs keeps the water out, but I think the former. You can stick your tongue out. Yeah, you’ll get water at the back of your throat, but as long as you don’t let it panic you, it won’t go down.
As for under water at deeper depths, well, if you’re using scuba equipment, the air inside you is already compressed versus the outside water, i.e., at deeper depths you have the same, compressed volume of air as you would on the surface, but it’s denses – it’s more air; there are more air molecules. The deeper you go, the faster you use your tank air, because you’re breathing much more of it.
If you have ever gargled with mouth wash, you know that your throat can shut off and not allow things down it at the wrong time.
Ross in the absense of any parts which are not formed the same as most people, I think your wife’s issue is mental. Most little children hold their nose when they go under water. After they have spent a lot of time in the water, most of us learn that it is not necessary to hold our nose as long as our nostrils are lower than our face. Water does not run uphill. With that said, I have met some adults that still hold their nose. It seems they never got that comfortable in the water, and just can’t make that leap of faith.
FTR on my first dive after my check dive, I ate a banana at 25’. Not hard to do, but it was salty. 
I didn’t know, either. I never tried to open my mouth underwater, and just sort of assumed it wouldn’t be pleasant. Actually, I didn’t even assumed anything, I think I just never even considered opening my mouth underwater. Doing so seems completely counter-intuitive to me.
Hell, I’ve seen footage from the 1950’s of people sitting at a table underwater playing cards. I always wondered if that was trick photography or something else.
But the drag slows you down. 
Even a cleft palate, as suggested previously, wouldn’t prevent her from being able to open her mouth underwater. The inability to stay place one’s face in the water (nose down) is purely psychological; but, as a former scuba instructor, I can testify that it is a very strong one, any many people who are not comfortable in the water either a) want to breathe in through the nose, or b) feel as if they are drowning just by having water on their face or their head completely submerged.
When you dive underwater, your sinuses and lungs equalize their internal pressure against the ambient water pressure; in order to do this, the lungs contract proportional to the increase in pressure. You needn’t hold your nose or block your throat, and in fact, when scuba or surface-supplied air diving, it is strongly recommended that you exhale slightly while the mouthpiece is out of your mouth; not to prevent water from entering the lungs, but to keep the diaphram relaxed so that should you accidentially ascend the air in your lungs can escape as it expands with reduced pressure, the alternative being overpressurization of the lungs and subsequent arterial gas embolism, pneumothorax (collapsed lung), and mediastinal or subcutaneous emphysema (escaped air pockets in the chest cavity or under the skin). These aren’t generally of concern with breathhold diving or freediving as the air never exceeds lung volume at surface pressure, though there are other potential respiratory hazards with these activities, such as hypercapnia, excessive hypocapnia, and alveoliar hemorraging.
Overcoming the “breathing reflex” for someone who thinks that they have to hold their nose underwater is a matter of conditioning and desensitization, nothing more. Start with the face in the tub or sink, work up to standing in the pool and breathing through a snorkel and mask, and eventually you’ll be able to breath underwater sans mask or pinched nose.
In short, the reason your lungs don’t fill up with water is the same reason that a glass jar, placed upside-down in a full sink or tub, will retain its measure of air (albeit somewhat compressed) rather than allow it to leak out. As long as you sieze your diaphram (i.e. hold your breath) or exhale, and don’t panic, there is no reason you can’t sneeze, cough, laugh, eat, barf, blow air rings, stick your tongue out, and otherwise open your mouth under water. Physiologically speaking, anyone can do this. Psychologically, it’s another matter; I’ve had a few students who couldn’t (or perhaps, wouldn’t) exert the discipline to overcome their reflexes and fears, and unfortunately, little enough time and attention is paid to this skill in standard scuba training to make all divers comfortable breathing underwater.
Stranger
I almost completely panicked on my first (discovery) dive – it was totally, completely counterintuitive to me to continue breathing as I went underwater (this was an ocean cove, not a pool). I was thinking, “I shouldn’t have lied about the claustrophobia on the pre-dive questionaire.” The panic was completely something instinctual, and my rational mind didn’t want to hear any of it. It took several descents and working through some of the “skills” before my body started accepting that I knew what I was doing!
Of course now, there’s not any hesitation at all. My only problem will be finding some place to rent gear and dive when I get back home.
The counterintuitive urge can be pretty strong. I used to snorkel when I was a kid, but gave it up for about 30 years. I started again last year. The moment I got my face in the water, I choked, panicked, drowned, and died a horrible watery death. Or at least came close. It took me a good hour of trying again and again before I was able to take a few desperate breaths underwater. One I had convinced myself that I could breathe almost normally, all of my former skills came back to me, and I was diving like a pro.
I’m assuming the diver allowed the cleaning shrimps into her mouth and allowed them to clean her teeth.
Once you have oral sex with those Chinese underwater, they’ll never let go. Although you really shouldn’t be referring to them in those terms.
Oh…wait…
I’m curious also, Jinx: Have you ever been swimming? I’m really not trying to be rude, but I’m having a hard time imagining how someone could learn to swim and never have tried opening his/her mouth underwater.
One of the games my friends and I used to play at the pool was going underwater and shouting at each other, then coming back up and trying to guess what the other had said.
I was the opposite. I thought it was the neatest thing to breath underwater. I felt like I was in my own little bubble of existence, where air existed in water.
That’s what my mind was trying to tell my body, but my body didn’t agree! I grew up 1/4-mile from Lake Huron, and the only thing I ever wanted to do was be able to breathe under it. Now, I may get my chance!