1cm of water is the pressure required to raise the level of water in a tube by one centimeter. A manometer is of course just basically a tube full of water with lines on it.
Well, a hint on a Television program sent me to the web and some research and it turns out that 1cm of H2O is very close to one hectopascal in the "metric" system (actually S.I. system).
A hecotpascal is also very close to the meterological measurement of one millibar and the normal sea level air pressure is fairly close to 1000 millibars and thus to 1000 hectopascals as well.
So 1cm of water turns out to be about 1/1000 of the "normal" sea level pressure of our atmosphere.
If your hose blowing 10cm of pressure into your face feels fairly stiff (and it does to me) it is actually an increase of about 1% of standard sea level atmospheric pressure, which seems surprisingly small in that light.
I don't know the importance of this, but I found it interesting anyway...maybe some of you will too.
The above is my opinion. It is just possible that I may, occasionally, be mistaken.
I am neither a Doctor, nor any other kind of medical professional.
Everything put together sooner or later falls apart.