No problem sucking water so long as you are off the floor of the cowl by at least an inch. Just pop the cover and have a look and you will see the huge hole in the cowl floor, but sitting an inch proud of it, for the heater intake. How often have you had water pour inside your cab through the heater? I have had water over the bonnet and halfway up the windscreen nearly during a quick deep hole during a river crossing and still didn't get any in. Longer under water might be a problem though.
It is also a pressure area and not a suction one as mentioned above so you have no worries there. That is why you get higher airflow at speed through the vent system. Have tested it also with strips of wool and they get sucked downwards at speed on top of the vent showing airflow running inwards.
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