The following how-to is very dangerous. Please do not try it without proper training and a friend that knows CPR. You must also be in very good health.
It is nearly universally accepted that humans, unlike fish, cannot breathe underwater without lots of special equipment. I've found that this is nearly true, but not quite. It actually is possible to absorb oxygen from water. This will not allow you to swim underwater indefinitely, but with practice I have been able to remain underwater for 10-12 minutes.
Our lungs are remarkable organs with impressive filtering capabilities. Did you know they have an interior surface area of about a tennis court? They contain millions of alveoli, which are tiny bubble-like structures that accept carbon dioxide from the blood and exchange it with the oxygen from the fluid in your lungs. Of course, this process works best when the fluid in your lungs is a gas, but it also works with water. However, don't just jump into your bathtub and start breathing; it requires a special process to not drown.
I'm going to warn you again, do not try this unless you are certain you can be revived if you pass out!
The process I am going to describe allows you to first fill one lung, then the other with water. Once this is completed, you can continue exchanging the water in your lungs with fresh, oxygen rich water.
First, lie on your side in shallow water. I recommend a bathtub because the water is clean (it's going to go into your body!) and you won't have to worry about waves. Breathe out about half of the air in your lungs, and allow your mouth to drop below the surface of the water.
This next step is the hardest. It will also be incredibly frightening. This process is essentially what is done in waterboarding. You need to very slowly breathe water through your mouth into your lung. As the water drains down your throat and into your bronchial passage, gravity will pull it down into the lung on the side you are laying on. You must fight the urge to choke! Remember, at this point you still have a lung full of air so you cannot easily drown. Continue breathing in water while laying on your side. This will take some time when you are first starting, but you can exhale slowly to allow your lung with air to empty. Breathe air in slowly to fill it back up, then go back to breathing water. If you find that you are choking too much, get of the water and take a break. Calm down, and try it again.
You'll be able to feel the water in your lung, which is an incredible experience. There are very few people living that have felt this! You can tell when your lung is full because you will be able to feel it on the top of your lung. Don't worry if you don't fill it completely on your first go at it, this will come with practice. Do, however, make sure it is at least 3/4 full.
Next, hold your breath and turn over onto your other side. Repeat the process of breathing water in slowly. You won't be able to stop and breathe air, but you've already had practice and you should be able to go faster. Also remember that your other lung is separating oxygen from the water and letting it into your blood stream. You will feel like you need air, but the feeling will pass. Continue breathing in water until this lung is full too.
There! That was definitely the hard part. Now you can exhale part of the water in your lungs and inhale new water. This will require more effort than breathing normally, as water is more dense than air. After enough practice, your diaphragm will get stronger. I don't recommend expelling all the water in your lungs right now. Until you are used to it, if your lungs empty it will feel like you are drowning all over again. Breathe out about half of the water, and breathe fresh water back in. Continue doing this until you feel comfortable with it. Your lungs will take longer than normal to release the oxygen from the water so take slow, deep breaths.
There you go, you're breathing underwater! In theory, you can keep this up as long as the water continues providing more oxygen than you are expending. That's why it's important to limit your physical activity when starting out with this skill. If you are practicing in your bathtub, you'll need to stop before you've extracted most of the oxygen from the water. This won't be a problem if the water is continually replaced, such in a lake or river.
When you're ready to go back to breathing air, exhale all the water in your lungs while you are still underwater. Bring your mouth back up over the surface, and take in a small breath. Resist the urge to take in all the air you can - even a small breath will be some shock your lungs. Take progressively larger breaths until you are again breathing normally.
After several weeks of practice, I have been able to breathe underwater for about 12 minutes. I expect that it is possible to remain doing so as long as you want, but I've been taking it slowly until I get to that point. I have done some very light swimming and diving while breathing water, and I've found it works great. Be sure not to overly exert yourself! Remember, you are getting oxygen more slowly than you are used to. The immediate advantage of this method over Scuba is that you do not need to worry about decompression sickness as you aren't breathing compressed air that contains nitrogen. I hope to one day swim underwater as long as I want and reach depths previously out of our unassisted reach.
Please let me know of your successes! I would love to hear about them, particularly if you can keep it up longer than I have!
This write-up is entirely fictional. I have not attempted to breathe underwater, and neither should you.