I have seen it a few times for vacuum cleaners.

Anyways, the conversion is easy: 1 mmH₂O is g Pascals, g = 9.80665, the Earth gravity, often rounded to 10.

It is actually easy to calculate:

1 Pa is 1 N/m²

1 N is the force needed to accelerate 1 kg at 1 m/s², the acceleration due to Earth gravity is "g", so, because F=ma, 1 kilogram-force under Earth gravity is g N.

Density of water is 1 kg/L, a column of water 1mm high with a 1m² base has a volume of 1/1000 of a m³, or a liter, so that column weights 1 kg.

Therefore that column exerts a force of g N on 1m², or g Pa. So 1 mmH₂O = g Pa =~ 10 Pa = 1daPa

I have never seen the daPa used as a unit, but the daN is common, because 1 daN so close to 1 kgf (kilogram-force).

Nearly all case fan brands use mmH₂O. No idea why, but that's what they do.

mmHg is reasonably common because that's how the first pressure gauges worked, and measuring how much a pressurized gas or liquid can move mercury against gravity is easy and reliable enough that this style stayed in use for 300 years [1]. But 0.8mmHg isn't a very impressive number, so at some point someone got the bright idea to use mmH₂O instead.

[1] https://en.wikipedia.org/wiki/Mercury_pressure_gauge

Inches of water is sometimes seen for retail vacuums in shop & household use. I have no idea where it came from - perhaps marketing where it gives a buyer an easy to visualize physical model for comparative purposes.

US doing everything possible to not use the metric system

He walked into a bar.

...I'll see myself out.

Someone said hectopascals and was, unfortunately, misunderstood.

... As per commenter below, will also see myself out.