This is important!
Colon absorbs water. Kidneys filter excess water 1000ml/hour. If your intake is more than that, you're going to end up with hyponatremia. It's called water intoxication and it is not a joke. Cardiac arrest, brain damage will ensue.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1770067/Understand that if you put in as "little" as 5 liters or more within short timespan, you're on borrowed time. That is, either the water comes out or you die. If you get bradycardia as a result of sudden electrolyte imbalance (say, hold the water in between filming shots), and fall unconscious, you're good as dead, because the water is not coming out, not fast enough.
Now tearing an intestine with air pressure will kill you too if the pressure is high enough to rip its way to the thoracic cavity (quickly) or destroying internal organs/causing peritoneal infection. The bacterial mass gets blown all over the peritoneal cavity and it is game over. It goes septic so fast nothing can be done.
You may think you're fine, because you've done it before, but what would happen if you had intestine wall weakened by say diverticulitis? Or any other inflammation/infection?
This is more dangerous than just about any fetish out there. Cardiac arrest can kill you standing up. You hit the ground lifeless. If you do it while holding liters of excess water entering your blood stream gradually, and they don't know what the fuck is happening, it doesn't matter if you collapsed inside ER. If by miracle they managed to keep your heart going, you'd probably still end up braindead via brain swelling.
This is not moralfagging. Air inflation in moderation is probably not dangerous. _But even relatively modest water inflation can kill you._ Massive air inflation can kill you too. Educate yourself.
Please spread this info around to whom it may concern.