Waterboarding Physics

In Scylla’s waterboarding thread, he asserts that as long as your lungs are above the water level you aren’t in danger of drowning. Is this true? Obviously you can drink a glass of water or breathe if you’re upside down, what prevents you from drowning?
Other websites I came across have methods where the water actually goes into the lungs but then is drained back out. What sort of damage could this do?
If it really isn’t dangerous why aren’t we doing lung enemas to help smokers clear out their lungs?
If I caught Scylla’s thread in the first few pages I would’ve asked there but it seems a too late to hijack in that direction now.