That pressure gauge is used for HVAC and needs to measure how much vacuum a pump is pulling in a line, how close to ZERO pressure, ie vacuum.
10meter water level pressure = 1 bar
Why would that gauge return pressure minus 1 atmosphere?
Same way I learned in school:
P = a+l*g*h
P = water pressure (hydrostatic)
a = atmospheric pressure at water's surface
l = water density
g = gravitational acceleration
h = water height
9.81 meters under fresh water the pressure should be 2 bars on a 1 bar atmosphere day.
I live in close to 1 bar atmosphere here at over 1200 ft alt (more than 300m). But today we have high pressure weather, 30.2 inches with standard day at sea level at 29.9 inches.
To get water depth right regardless of weather/altitude you will need two gauges. One just for the correct air pressure as that pushes down on the water and ADDS to water pressure.
That should change your scale at the very least.